Smithsonian Report/1898/The Tundras and Steppes of Prehistoric Europe

by James Geikie

 

THE TUNDRAS AND STEPPES OF PREHISTORIC EUROPE.^[1]

 

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By Prof. James Geikie, D.C.L., LL.D., F.R.S.

(With map.)

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I.

We are all familiar with the general conclusion arrived at by geologists that our earth has experienced many climatic changes. There have been times when genial conditions ranged up to the highest latitudes, and times also when the cold of the arctic regions descended to what is now our temperate zone. The cause or causes of those remarkable vicissitudes still baffle research. Many explanations have been advanced—some highly improbable, others perhaps more likely, while of yet others it may be said that possibly they contain a certain amount of truth. But no one theory or hypothesis has succeeded in gaining general assent, and we shall not therefore at present concern ourselves with any. In place of reviewing hypotheses and speculations, I shall limit myself to a survey of certain facts connected with the later geological history of our continent, the meaning of which is more or less apparent. The evidence referred to leads to the conclusion that Middle Europe has within the human period experienced conditions such, as now obtain in the tundras and barren grounds of circumpolar regions. When these conditions passed away, the central and west-central areas of our continent became steppe lands, comparable as regards climate to the subarctic steppes of southeast Russia and southwest Siberia.

As geologists reason from the present to the past, it will be well to take first a brief glance at those regions of the globe where at present tundra and steppe conditions respectively prevail. When we have realized the salient characters of those regions, and the nature of their floras and faunas, we shall be in a better position to understand the bearing of the geological evidence.

The arctic lands of Eurasia and North America show two well-marked zones—a zone of treeless wastes bordering the Polar Sea, and a coniferous forest zone lying immediately to the south. The treeless wastes are known as tundras in Europe and Asia, and as barren grounds in North America. These form plains of immense extent, but of very unequal width from north to south. In Eurasia they lie for the most part north of the Arctic Circle, while in North America they range upon the whole considerably farther south, reaching the sixtieth parallel on the western shores of Hudson Bay. Their southern boundary, however, is in both Old and New Worlds exceedingly irregular. Where the flat lands are exposed to the full sweep of the northern blasts, tundra conditions advance far to the south, invading the forest zone in narrower or broader stretches. Indeed, even within the region of arctic forests isolated patches and wider areas of tundra are encountered. In other places more sheltered from the fierce winds coming from the polar seas, the arctic forests in their turn encroach upon the tundras, so as nearly to reach the shores of the frozen ocean. Such is the case in the valleys of the Yenesei, the Khatanga, the Olenek, the Lena, and other North Siberian rivers. Similarly in North America the arctic forests straggle down the valleys of the Mackenzie and other rivers to beyond the Arctic Circle.

Mosses and lichens form the prevailing vegetation of the tundras— marshes and bogs extending over vast areas in spring and summer, while the less marshy tracts are carpeted with gray lichens. Here and there, too, in sheltered spots, dwarf birch and willow scrub sprinkle the surface or flourish in denser masses, and ever and anon more or less wide stretches of meadow put in an appearance. Now and again the interminable plains give place to rolling ground, the low hills and knolls being not infrequently clothed with dwarf trees. No hard and fast line, indeed, can be drawn between the tundras and the arctic forests. The two regions not only interosculate, but numerous oases of trees are encountered in the tundras along their southern margin, while equally numerous patches of tundra, as already mentioned, are met with farther south within the arctic zone. It may be added that in northern Siberia bare rocky hills and mountains—highly fissured, and showing many gullies, ravines, and debris-strewn valleys—now and again break the uniformity of a tundra landscape.

A word or two now as to the characteristic animals of the tundras and barren grounds. First among these come the arctic lemmings. They feed on grass roots and stalks, mosses, reindeer linchens, and the shoots of the dwarf birch, for which in winter they tunnel through the turf or under the snow. The banded lemming is an especially characteristic form, since it is confined to the maritime tracts of Eurasia and the adjacent islands, and the corresponding areas of North America, and is never met with in the forest zone. The Obi lemming has a similar distribution, but ranges somewhat farther south, and not quite so far north, as the banded lemming. The arctic fox is another characteristic member of the tundra fauna, having a high northern range. It occasionally wanders south to the sixtieth parallel, but that is only in treeless regions, for it everywhere avoids the forest, seeming to prefer the barest and most sterile lands. Another common denizen of the tundras is the arctic or mountain hare. This is the same species so commonly met with above the limits of the forests in the mountains of temperate Europe. A closely allied form (polar hare) frequents the barrens of North America. The reindeer must also be included in the tundra fauna, although in winter it ranges far into the forest zone. The muskox, formerly a native of Eurasia, is now confined to North America. Like the arctic fox it avoids the forests, ranging north of these from the the sixtieth parallel up to the highest latitudes.

Such are the most characteristic mammals of the tundras. There are many other animals, however, which frequent the same regions, more especially in summer. Among these may be mentioned glutton, voles, ermine, weasel, wolf, common fox, and brown bear. The summer visitors also include a vast host of birds, especially water birds.

The climate of all these northern plains is extreme—the winter temperature falling upon an average to 27° below zero, while in summer the average temperature is about 50° F. The actual range in certain regions is of course considerably greater. These conditions necessarily give rise to annual migrations. Only a few mammals, as we have seen, brave the long winter of the tundras, where river and lake are often frozen solid, and the whole land is sheeted in snow. During the great frosts the air is remarkably still, but as winter draws to a close storms of wind and snow become frequent. Wide regions are then often swept bare, and the snow is blown into every abrupt hollow and depression in the plains, and into the gullies and ravines of the hills, where it becomes so beaten as often to bear the weight of a man. Not only snow, but sand and dust, are thus swept forward. The sand and dust are no doubt largely obtained from the great river valleys and deltas, but no inconsiderable proportion is derived also from the bare rocky hills and mountains, which in many places diversify the surface of the circum- polar plains. Frost is a great pulverizer of rocks, not only splitting them into fragments, but disintegrating their surfaces into grit, sand, and dust. It is remarkable how in the highest northern regions the surface of the snow often becomes discolored with fine sand and dust derived in this way from exposed rock surfaces.

We need not enter into further details as to the physical conditions of the tundras. It will be sufficient to sum up here the points which are most deserving of our attention. Briefly they are these:

1. The climatic conditions of the tundras are extreme, and necessitate annual migrations.

2. The flora is represented chiefly by mosses and lichens. Here and there, however, tracts of grassy meadow occur, while inlets and oases of dwarf trees and scrub, chiefly birch, willow, juniper, and conifers, not infrequently appear along the southern margin of the tundras.

3. The most characteristic animal forms are lemmings, arctic fox, arctic hare, musk ox, and reindeer. Of common occurrence also are various voles, ermine, and weasel. Their range, however, is hardly so far north, and they go much farther south. So again the wolf, the ubiquitous common fox, and the brown bear, are frequent visitants rather than common denizens of the tundras.

4. In summer many of the animals just named push farther north, while swarms of birds (especially water birds) visit every part of the treeless zone.

5. Lastly, in winter, storms of snow and dust are common.

We may now take a similar brief glance at the steppe lands of Europe and Asia. The regions included under this head show considerable variety. Some steppes are mere desert wastes while others are fertile tracts capable of high cultivation. Many are low plains, others are elevated plateaus, the former having a subarctic, the latter a subtropical climate; and between low and high steppes many gradations are met with. All are more or less characterized by an extreme range of temperature. The steppes with which we are at present concerned, however, are the generally low grassy plains which Professor Nehring designates the subarctic steppes. These occupy wide areas in south- east Russia and southwest Siberia, extending between the middle course of the Volga and that of the Irtysch. It is quite a mistake to suppose that these steppes are throughout all their extent treeless plains. In many places chains and irregular groups of hills diversify the surface, while here and there trees of various kinds, such as pines, larches, birches, oaks, limes, alders, willows, wild apples, and others, are more or less plentiful. Many of the woods are mere oases, extending along the banks of rivers and streams, or clustering around the margins of fresh-water lakes. In southeast Russia the boundary between the steppes and the forest lands is very irregular—the two regions constantly interosculate.

The climate of these subarctic steppes is quite continental, the summer being relatively warm and the winter relatively cold. The average temperature in January hardly exceeds 3° F. while that of July is at least 70°. Again, the rainfall is very uncertain. In some years it is excessive, in others meager, while occasionally it altogether fails. With the approach of spring vegetation rapidly develops, becoming rank and luxuriant, but with the heat of summer it quickly fades and withers away. Severe frost, and frequently heavy snowstorms, characterize the winter. In such areas as are more or less wooded the climate is somewhat less continental, the summers being relatively less dry and the winters not so cold. But even in those wooded regions the seasons are strongly contrasted. In general, we may say the steppe lands in summer are practically rainless. The ground is thus parched and burnt up, so that sand and dust rise with every wind, and as the open plains are often swept by summer burans, vast quantities of loose materials are transported from place to place, and here and there accumulate in hollows and depressions, or come to rest in the lee of sheltering rocks and hills. In winter, if little snow has fallen, the unprotected ground is similarly scoured by the tempests, dust, sand, and even small stone being carried forward. Thus both in summer and winter sand and dust storms play an important role, and loose materials are piled up to great depths in valleys, and in the ravines, fissures, and crevices of the rocky hills.

As a rule these heaps and sheets of drifted sand and dust show little or no arrangement, although now and again some trace of bedding may appear. Should they chance to become well covered with snow in winter, then, when warmth returns and the snow gradually melts away, plants quickly spring up, and the heaps become fixed and cease to drift. It is obvious that not infrequently land shells, and often enough the remains of mammals, must be entombed in such wind-blown materials.

In winter, however, it is snow more commonly than dust that drifts before the wind. The great snowstorms of the subarctic steppes are quite as terrible as those of the tundras. No life can withstand the fury of the blizzards, and many are the disasters on record. Thus in 1827 all the flocks and herds that wandered over the steppes between the Volga and the Urals perished in one great storm. According to the Government report the loss sustained by the Kirghiz amounted to 10,500 camels, 280,500 horses, 30,480 cattle, and 1,012,000 sheep. Not many years pass without some disaster of this kind, and when the snow has melted away, hundreds of cattle, often far strayed, may be found huddled together in one place—some suffocated, frozen, or starved to death, others drowned in the creeks and ravines in which they had vainly sought for refuge from the blast. Now we can readily conceive how the carcasses might eventually be buried under drifted sand and dust, and the bony skeletons thus become preserved for an indefinite period.

Among the most characteristic animals of the subarctic steppes are jerboas, pouched marmots, bobac, pika or tailless hare, small hamster rat, various voles, corsac, caragan fox, manul cat, saiga, dzeggetai, wild horse, etc. Besides these, many other animals are met with in the steppes, but are hardly so characteristic, since they range into adjacent regions, to which they more properly belong. Among them may be mentioned lynx, wild-cats, tiger, wolf, jackal, common fox, martens, ermine, weasel, otter, glutton, badger, brown bear, squirrels, beaver, common hare, mountain hare, wild boar, elk, reindeer, roedeer, stag, etc. Several hundred species of birds frequent the steppes, among which may be mentioned great and little bustards, larks, grouse, buzzards, eagles, owls, etc.

We may now sum up, in a few words, those features and characters of the subarctic steppes which are of most importance from our present point of view.

1. Steppes, like tundras, are not exclusively plains. They include rocky uplands and hills, and are traversed in many places by streams and rivers.

2. Vast expanses are clothed with grasses, while others are more or less sterile and bare. Oases of forests are not infrequently present.

3. The most characteristic animals are jerboas, pouched marmots, bobacs, and others—the mammalian fauna being more varied than that of the tundras.

4. Many animals properly belonging to forest lands and to mountains frequent the steppes.

5. The seasons are strongly contrasted, and the whole region is exposed to dust storms in summer, and to snowstorms in winter.

With these facts relating to existing tundras and steppes kept in view, let ns now examine the evidence adduced by geologists to show that tundra and steppe conditions have successively prevailed in Middle Europe.

One of the most remarkable superficial deposits of central and west- central Europe is that which is known under the general term of löss. Typically it is a fine-grained, yellowish, calcareous, sandy loam—consisting very largely of minute grains of quartz, with some admixture of argillaceous and calcareous matter. Upon the whole the quartz grains are well rounded, although often enough they are sharply angular. Frequently the accumulation shows a porous structure, and is penetrated by long, approximately vertical root-like tubes or canals, lined with calcareous matter, which cause the deposit to cleave or divide in vertical planes. Hence it usually forms more or less upright bluffs upon the margins of streams or rivers which intersect it. It is usually unstratified, except now and again toward the bottom of the deposit, where intercalated layers, and even sometimes thick beds of sand, make their appearance. The loss is essentially a deposit of the low grounds, and is well developed in the broad river valleys of western and central Europe, as in those of the Seine, the Garonne, the Rhone, the Maas, the Moselle, the Rhine and its tributaries, the Danube and many of its affluents, such as the Drave, the Save, the Morava, and the Theiss. It also extends as a narrow belt along the southern margin of the great plains of North Germany. It is in southern and southeastern Russia, however, where it attains its widest development, covering as it does an immense tract, stretching west and east between the valleys of the Pruth and the Volga. Throughout this vast region it is usually very dark in color, forming what is known as the black earth.

Without at present going into the question as to the origin of the materials of which the loss is composed, it is obvious enough that they have in some places been arranged by water. Thus here and there, especially at or toward the bottom of the accumulation, distinct traces of bedding may be seen, and the beds have yielded fresh-water shells. This, however, is exceptional. Löss is, for the most part, a subaerial accumulation—a wind-blown deposit. This is shown not only by the rounded character of its minute constituents and by the general absence of bedded arrangement, but by the abundant presence of snail shells and the frequent occurrence of relics of land animals. Its organic remains are essentially terrestrial. Moreover, its particular distribution—the mode in which it occurs — points clearly to the action of prevalent winds. Thus, although it is widely developed over low-lying regions, it nevertheless sweeps up to heights of 200 to 300 feet and more above the bottom of the great river valleys. Not only so, but ever and anon it extends across the hills and plateaus between adjacent valleys, wrapping the whole land, in short, like a mantle. Again, in many places, we find it heaped up in the lee of hills, the exposed windward slopes of which bear no trace of it, while in certain valleys it shows a similar partial distribution.

Among the organic remains yielded by the löss are some that indicate arctic conditions, while others are strongly suggestive of a steppe climate, and yet others tell us of forest lands. It is impossible that all the creatures referred to could have lived side by side in the same region, and annual migrations will not wholly explain their appearance in the same deposit. The evidence leads to the conclusion that the accumulation of the löss must represent a long period of time daring which climatic changes took place. Fortunately now and again the lössic accumulations exhibit a succession of faunal zones—different suites of organic remains occurring at different levels. And a similar and corresponding succession has been discovered in many of the caves of middle Europe.

A tundra fauna is the earliest of which we have any record in the löss and in the particular caves referred to, and it is worth while to glance for a moment at the former wide distribution of that fauna in Europe. It will be remembered that two of the most characteristic tundra forms are the banded and the Obi lemmings. Now, remains of both these species have been met with again and again over all central Europe—in Russia, Poland, Austria-Hungary, north and south Ger- many, north Switzerland, France, Belgium, and England. Sometimes they occur in single specimens, at other times they are extremely numerous, the remains of several hundreds having been obtained at various localities. In many places both species of lemming are found together; elsewhere either one or other occurs alone. The banded lemming, as a rule, has left its remains most abundantly in hilly and upland tracts, while those of the Obi lemming are met with more frequently in low-lying areas—a distribution quite in keeping with that which obtains at present in the tundras. That these arctic animals were not mere passing or occasional visitors is shown by the fact that young and full-grown individuals occur together in hundreds at various places and are associated with the remains of other characteristic arctic animals which breed in the same regions. Thus well-preserved skeletons of arctic fox, having their milk teeth, have been found lying side by side with the bones of the lemmings. As the arctic fox breeds in June, it is obvious that those young individuals must have died in summer.

Our knowledge of the former distribution of the arctic lemmings is no doubt not so full as it will yet be, but already we have ascertained that these creatures ranged as far south as central France and the base of the Alps, in Switzerland, and as far west as Somerset, in England. Besides the arctic fox, many other northern forms were congeners of the lemmings in middle and western Europe, such as mountain hare, muskox, reindeer, glutton, voles of various kinds, ermine, weasel, wolf, common fox, and the now extinct mammoth and woolly rhinoceros. A number of northern birds have also been recorded from the same deposits as those which have yielded relics of the tundra animals. I need mention only ptarmigans, buntings, snow owls, ducks, geese, and swans, all of which are in harmony with the arctic character of the mammals, since the same forms are in our day constant summer visitants in the circumpolar treeless lands.

We may note, further, that just as there is this evidence to the former occupation of middle and western Europe by an arctic fauna, so we have abundant traces in the same regions of a well-marked arctic flora. High northern species of mosses, the polar willow, the dwarf birch, and various other northern plants have been met with in superficial deposits over a very wide area, extending from southern Sweden and England across middle Europe to the foot of the Alps.

We can not doubt, therefore, that true tundra conditions have formerly prevailed at relatively low latitudes in Europe. The widespread distribution of the arctic animals and plants just mentioned points clearly to that and to no other conclusion. We may therefore reasonably infer that the climate of middle Europe must then have approximated in character to that of northern Siberia, the seasons being doubtless strongly contrasted, and thus compelling annual migrations. With the advent of summer the home of the arctic lemmings was invaded by troops of visitants—by mammoth, woolly rhinoceros, wild horse, saiga, and many others, and by numerous birds. An arctic-alpine vegetation clothed the low grounds, which in the warm season doubtless showed wide stretches of bog and marsh and many shallow lakes. Here and there flourished patches and wider tracts of birch and willow scrub, but the land was practically treeless. Man, we know, was an occupant of middle Europe at this time. Perhaps, like the mammoth and the woolly rhinoceros, he may have been rather a summer visitor than a constant denizen, departing for more clement regions at the approach of winter. We shall probably not err in supposing that the winter would have much resemblance to that now experienced in northern Siberia—long spells of still weather, with intense frost, interrupted now and again (especially at the changes of the seasons) by fierce snowstorms, in which the wild animals could hardly fail occasionally to perish in large numbers.

How long these tundra conditions obtained we can not tell. All we know is that eventually they gradually passed away and the climate became less arctic. This is shown by the well-ascertained fact that both in the loss and the contemporaneous cave accumulations remains of the arctic animals are confined to the lowest beds, becoming gradually less numerous as we trace them upward, until they finally disappear. But before the last of the tundra forms has vanished remains of a steppe fauna begin to occur. In a word, there was no sudden dying out of one fauna and precipitate appearance of another, but a gradual replacement, consequent, doubtless, upon changing climatic conditions.

All the animals already mentioned as most characteristic of the subarctic steppes are represented in the caves and alluvial deposits of west and middle Europe. Jerboas, pouched marmots, bobacs, and true marmots, tailless hares and others, all formerly flourished in those latitudes. Besides these most characteristic steppe animals occurred many other forms which were not restricted to steppe lands, such as mammoth and woolly rhinoceros, marsh lynx, cave lion, hyena, wolf, common fox, ermine, weasel, badger, reindeer, urus, bison, etc. Many birds also were present—all of them species which in our own day frequent the steppes of southeast Russia. Land shells are also very often found in less or greater abundance along with the relics of the steppe animals just mentioned, most of the shells representing forms that now live in dry steppes, while some are denizens of wooded regions.

The plant remains associated with relics of the steppe fauna are quite in keeping with the latter, but are upon the whole seldom met with, the conditions not being favorable to their preservation. Trunks and branches of trees occur very rarely, the most common remains being a few thin layers and seams of peaty matter, apparently consisting chiefly of grasses. Nevertheless, we need have no doubt that a steppe flora formerly flourished in middle Europe, for (as Engler, Ascherson, Petry, and other botanists have shown) many well-known steppe plants survive in the existing flora of that region.

Among the animals associated with the true steppe forms were some which, as we have seen, had already invaded central Europe in tundra times. Of these, perhaps the most notable are the mammoth and the woolly rhinoceros. Probably they were only summer visitors, but in the subsequent steppe epoch they became truly indigenous and very abundant. The broad valleys and open spaces of central Europe were at that time treeless plains, although woods seem to have existed here and there, especially along the margins of lakes and streams. The climate, we need not doubt, was much like that of the subarctic steppes of southeast Russia and southwest Siberia, regions which, like the tundras, are much exposed to wind action. The general character and distribution of the löss prove its æolian origin, and its organic contents are quite in keeping. We may be sure, then, that dry steppe conditions formerly prevailed throughout central Europe, and that in those regions dust storms and snowstorms must have been of common occurrence. We have seen how, in existing tundras and steppes, the semidomesticated and wild animals of those regions are now and again overwhelmed in storms and smothered in snow. Now, similar catastrophes must have happened again and again in the tundras and steppes of prehistoric times. And we are not left in this matter to mere conjecture, for the carcasses of some of the more notable animals of those days, now extinct, have been preserved to the present in the frozen snows—the famous ice formations of northern Siberia. So perfectly preserved, indeed, was the mammoth discovered by Mr. Adams that its flesh was devoured by wolves and bears, and from the appearances presented by it and others we can not doubt that the animals had perished in snowdrifts. Brandt records, for example, that the congested veins and capillary vessels in the head of a rhinoceros examined by him were charged with coagulated blood, as if the animal had died of suffocation; and Schrenck says of another described by him, that the distended nostrils and gaping mouth were highly suggestive of a similar death. It is probable that these animals were summer visitors to the tundras, overtaken by autumnal snowstorms. If perfectly preserved carcasses are rare, such is not the case with skeletal remains. In many places throughout Siberia the bones of various mammals occur in enormous quantities, huddled together, as it were, in very limited spots. It seems impossible to account for such hecatombs on any other supposition than that they are the silent records of great blizzards and snowtorms. Even in our own time herds of wild reindeer, with their young, are overcome by snowstorms in the tundras, while in North America great flocks of sheep and cattle frequently perish in the same way. Professor Garman, who draws attention to the disastrous results of blizzards in the great prairie lands of that region, is of opinion that the extraordinary heaps of skulls and other remains of the bison that are met with here and there in northern Colorado and Wyoming, are the remains of herds which have been suffocated in snowdrifts.

It is not necessary to suppose that all the relics and remains of the mammoth and its congeners in Siberia are evidence of the destructive effect of blizzards. The animals doubtless met their death under many different circumstances. Sometimes they would appear to have been bogged in swampy holes and morasses. I have referred to the peculiar ice formations of the arctic coast lands. These are sheets of ice of unknown thickness, preserved under more or less thick accumulations of earthy and loamy materials. The ice is believed to represent the blown or drifted snows of prehistoric times, which here and there have been protected from complete dissolution by soil and subsoil flowing over and accumulating upon them, under the influence of thaw, in spring and summer. Such movements of superficial materials are indeed of common occurrence in high latitudes at the present day. The surface of the buried ice strata is very uneven, being furrowed and trenched by deep ruts and hollows. These depressions are filled up with frozen mud, etc., containing vegetable debris and abundant mammalian remains, including those of mammoth and woolly rhinoceros. Probably a large number of the bones may simply have been introduced into the hollows by the flowing soil in spring—they may have been lying originally scattered over the surface. In other cases, however, the animals themselves seem to have fallen or sunk into the depressions. All the evidence leads to the inference that in the warm season these high northern regions were visited abundantly by mammoths, rhinoceroses, horses, bisons, wapiti, and others. Such being the case, it is not hard to understand how the bulkier animals might now and again become trapped in the treacherous bogs and subjacent muds that covered and concealed the ice formations and their deep clefts and depressions.

When we turn to the loss of Europe, we meet with copious evidence to show that the wild animals of our prehistoric steppes and tundras were often done to death in their hundreds and thousands. Again and again great heaps and accumulations of their skulls and skeletal remains have been encountered in our lössic accumulations—appearances exactly recalling the similar bone finds of Siberia and North America. The deposits in which the European bone finds occur are of wind-blown origin, and we seem justified, therefore, in concluding that the animals perished in snowstorms. In these low latitudes, however, we could not expect to meet with ice formations like those of the tundras. But that drifted snows did formerly accumulate in middle Europe, and were preserved for long periods under coverings of sand and other materials, we have good reasons for believing. Indeed, even at the present day the drifted snows in southeast Russia are occasionally buried under sand and so persist for years. In one ease recorded by Borszcow, what appeared to be an ordinary sandhill proved to be a mass of congealed snow cloaked in sand about a foot in thickness. Immediately under the surface the snow was granular and neve-like, but a little deeper it was firm and solid like ice. This was in one of the tributary valleys of the Ilek, in the steppes south of Orenburg, about the fiftieth parallel—a relatively dry region. If in a low-lying region so far south snow can be preserved in this way, we may readily believe that in the steppe epoch of middle Europe snowdrifts similarly protected might now and again have persisted for years. But it was during the preceding tundra epoch that this would be most commonly the case. And much interesting evidence is forthcoming to show that in many places thick sheets of congealed snow did accumulate and become buried and preserved at that time. Many of the so-called "rubble drifts" of middle Europe—sheets of rocky rubbish which have traveled down gentle hill slopes and spread themselves over the adjacent low grounds—point to the former presence of great snow drifts, in and upon which the rock debris traveled. These were not glaciers, but simply sheets of neve-like snow, charged with and covered by earthy and rocky debris, which kept moving outward, more especially in spring and summer when the heaps were more or less rapidly melting. The occurrence in this debris of bones of the reindeer and other mammals shows that the deposits belong to prehistoric times. Again, certain phenomena connected with the river gravels of the same period lead to the conviction that the drainage was often interfered with by snowdrifts in tundra times. The river valleys would seem to have become filled in places with alternate sheets of congealed snow or ice and layers of gravel and shingle. Long afterwards, when the interbedded strata of ice melted slowly away, the associated river detritus quietly settled down, and owing to the differential movement of the subsiding materials the longer stones naturally arranged themselves in lines of least resistance, so that now we find them most usually standing on end in the gravel beds.

Thus, apart from the evidence supplied by the bone accumulations of the loss, we have good reason to believe that snowdrifts were of common occurrence in middle Europe in prehistoric times. Doubtless most of the snow which covered the plains of our continent in winter melted and disappeared in summer, just as is the case in the tundras and steppes of our own day. The carcasses of animals that may have perished in blizzards would thus most frequently become uncovered in spring, to be devoured by hyenas, wolves, and bears, and the disarticulated skeletons might often be bleached and weatherworn before they were finally buried in loss. Nor was it only in plains and open valleys that sudden death may have overtaken large numbers of animals at a time. In tundras and steppes alike the wild and semiwild denizens of the plains seek refuge from the drifting snow in the fissures, caves, gullies, and ravines of the hills and mountains, where they are sometimes frozen to death or smothered in snow. Herbivorous and carnivorous animals thus often perish together, for in the presence of a common danger, whether it be prairie or forest fire, or flood or blizzard—natural antipathies and animosities are forgotten, and all alike struggle to escape.

Man, as I have already mentioned, lived in middle Europe in tundra times, and we have abundant evidence of his presence there throughout the succeeding steppe epoch. Again and again his relics and remains have been met with at all levels in the loss throughout central Europe. Thus in the valleys of the Danube and some of its tributaries they have been discovered in undisturbed löss at depths of from 20 to nearly 100 feet from the surface. Not a few of these finds evidently represent old prehistoric camping stations—marked by the presence of quantities of charcoal and ashes, burnt and calcined bones, together with worked flints, bones, and ivory. Among the animal remains are those of mam- moth, woolly rhinoceros, musk ox, reindeer, elk, horse, lion, glutton, bear, wolf, arctic fox, common fox, and hyena. Nor is it only in the loss that we have human relics associated with the tundra and steppe faunas. Similar finds have been recorded from many caves and rock shelters, of which we may take the rock shelter of the Schweizersbild, near Schaffhausen, as a good example. The deposits at that place show a clear succession, and tell a highly interesting tale. The following is the sequence, the beds being numbered from below upward :

6. Humus bed.
5. Gray relic bed.
4. Breccia bed, with upper rodent bed.
3. Yellow relic bed.
2. Lemming bed.
1. Gravel bed.

With the lowest bed (No. 1) we need not at present concern ourselves, beyond remarking that it is obviously of fluviatile origin. All the overlying beds are clearly of subaerial formation—the flooded torrential water, which laid down the gravel bed (No. 1), had left the rock shelter high and dry before the succeeding lemming bed began to accumulate. This latter is a yellowish earth, charged with fragments of limestone detached by the weather from the overhanging rock. Scattered through this earth are abundant remains of arctic lemming, arctic fox, mountain hare, reindeer, glutton, and a number of other forms which are constant summer visitors to the tundras. The banded lemming is the most plentifully represented species, and next to it in abundance comes the alpine hare. In close association with this tundra fauna occur flint implements, and awls, chisels, harpoons, and needles of bone and horn. Only one old hearth, with its ashes, was encountered, and from the fact that no calcined bones were met with, while the number of worked bones and antlers was relatively small, it may be inferred that man was not a persistent occupant of the rock shelter during the slow accumulation of the lemming bed. The same conclusion is suggested by the occurrence, especially in the upper part of the bed, of abundant traces of various birds of prey, which appear to have been able to nest undisturbed on the rock and in its crevices.

It can not be doubted, therefore, that during the formation of the lemming bed an arctic climate reigned in north Switzerland. Toward the upper part of that bed, however, we find evidence to show that tundra conditions were gradually passing away. This is indicated by the fact that some of the tundra animals, so common in the lower part of the stratum, become scarcer, and at last cease to appear, while at the same time a few representatives of the subarctic steppe fauna enter upon the scene.

The next succeeding stratum (yellow relic bed) proved to be rich in human relics. It yielded some 14,000 flint implements, and a large number of worked bones and antlers, comprising needles, bodkins and awls, chisels, harpoons, whistles, and other objects. Bits of wood worked and charred, and fragments of worked and unworked lignite were also obtained. Besides these, drawings and patterns were found on reindeer antlers, on bones, and on tablets of limestone, while many shells, fossils, and teeth of the arctic fox and the glutton were met with, bored and pierced, as if they had been used for necklaces and other personal ornaments. The presence throughout this relic bed of nuclei or cores from which flints had been struck, of abundant chips and splinters, of old hearths, ashes, and burnt bones, shows that the reindeer hunters were for a long time constant occupants of the rock shelter.

Turning to the abundant animal remains, we find that these represent no fewer than 49 species, viz, 30 mammals, 15 birds, 3 amphibians, and 1 fish. All the most characteristic tundra forms—the banded lemming and its peculiar associates—are now absent, and in their place we find a true steppe fauna. Amongst the new arrivals are red suslik, pika, and true hamster, and associated with these are such constant visitors of the steppes as manul cat, wild horse, dzeggetai, and various birds. Certain forms which appear in the lemming bed are still represented, as arctic fox, glutton, and others—all of which, however, in our own day range south of the true tundras. Their presence therefore is not out of keeping with the characteristic steppe forms. It is clear therefore that in north Switzerland a tundra fauna was eventually succeeded by a steppe fauna.

Toward the top of the yellow relic bed once more new arrivals begin to put in an appearance, and their presence seems to show that the climate was again gradually changing, for they include red deer, roe deer, wild boar, squirrel, pine marten, and beaver, all of which belong to a forest fauna.

The next stratum in succession is the breccia bed. This consists of small fragments of limestone, either lying loosely together or cemented by calcareous matter. Relics of man were not so common in this bed, although occasional splintered bones and flint implements occurred all through it, and in places were even abundant. About midway between the top and bottom of the breccia occurred a layer of dark-earth, in which human relics and the remains of various rodents were conspicuous. It would seem that during the accumulation of the breccia bed small groups of reindeer hunters only now and again visited the rock shelter; it was evidently not so continually occupied as it had been. The animal remains met with in the stratum undoubtedly tell a tale of changing climatic conditions. Amongst the species represented are reindeer, pika, hare, squirrel-tailed dormouse, garden dormouse, squirrel, water rat, various voles, shrews, mole, ermine, marten, and others. This is obviously a mixed fauna—a few of the steppe animals being still present, but the larger number of the species are forest forms. The fauna of the breccia bed, in a word, marks the transition from steppe to forest conditions. Obviously the climate was gradually improving, the forests continuing to increase at the expense of the earlier steppe flora.

In the gray relic bed that succeeds we lose all trace of the characteristic steppe fauna. The most abundant remains are those of red deer, roe deer, horse, and ox, and with these are associated relics of a number of other forms, such as badger, wild-cat, hare, urus, goat, and sheep. The steppe fauna had now obviously become replaced by a forest fauna. Paleolithic man—the reindeer hunter of the tundras and steppes—had also vanished, and his Neolithic successor now occupied the rock shelter of the Schweizersbild. The gray relic bed and the overlying humus bed tell a most interesting tale, but into that I can not go. It is sufficient to note that the old reindeer hunters seem to have departed before forest conditions had been fully established. We may surmise that as the climate became warmer the reindeer gradually withdrew from the Alpine Vorland. Probably it had already become somewhat scarce during the accumulation of the breccia bed, in which, as will be remembered, traces and remains of it and its hunters become less and less common. One can hardly doubt that the emigration of the reindeer and the final exodus of Paleolithic man from north Switzerland were contemporaneous events, brought about by changing climatic conditions. We can picture to ourselves the old race of hunters, with the contemporaneous steppe fauna, gradually passing east and northeast, while the forests continued to encroach upon and overspread the fertile lands of central Europe. It is possible that Neolithic man may here and there have come into contact with his Paleolithic predecessor, but of this we have no evidence. All we certainly know is that the latter vanished from central Europe with the steppe fauna, and that when Neolithic man made his earliest appearance a forest fauna was in possession of the land.

II.^[2]

In my preceding lecture evidence was adduced to show that tundras and steppes, with their characteristic faunas, formerly existed in central and west central Europe. We saw that for a long time the climatic conditions of these regions must have resembled those that now obtain in northern Siberia and the barren grounds of North America, where mosses and lichens form the prevailing growths, and arctic lemmings, hares, and foxes, the reindeer, and the musk ox are the common indigenous animals. All these characteristic species formerly lived in middle Europe. Eventually our tundra flora and fauna gradually disappeared and were as gradually replaced by steppe forms of life. Jerboas, pouched marmots, pika, and many others—such an assemblage as we now see in the subarctic steppes of southeast Russia and southwest Siberia—flourished throughout the regions over which the lemmings and their arctic congeners had formerly prevailed. Throughout both tundra and steppe epochs Palæolithic man was an occupant of middle Europe. To the steppe epoch succeeded a forest epoch, with its characteristic fauna, by which time Palæolithic man had vanished, his place being taken by the so-called Neolithic race, or races, for there were several of these.

We must now ask what relation the tundra and steppes deposits bear to other well-known superficial accumulations of Europe. To what particular stage of the geological history of our continent do they belong? When we remember that an arctic-alpine flora formerly flourished on the low grounds of central Europe, it seems extremely probable that the tundra epoch must fall within the glacial period. But the glacial period embraced a complex series of geographical and climatic changes, and it is necessary, therefore, to come somewhat closer to the question. Among the most conspicuous deposits of the Ice age are moraines of all kinds and fluvio-glacial gravels, while the löss, as we have seen, is the most prominent accumulation of the tundra and steppe epochs. How, then, does the latter behave with regard to the typical glacial and fluvio glacial formations? Is it older or younger than these, or are the two sets of accumulations contemporaneous? The answer we get to that question is, at the first blush, disconcerting, for we learn that it is each in turn—sometimes underlying, sometimes overlying, and in other places occurring intercalated among glacial deposits. This only means, however, that löss appears to have been formed during different stages of the Ice age. It will be remembered that while we discussed the wind-blown character of the loss, we left untouched the question of the origin of its materials. Whence were those materials derived which the wind worked over, and largely rearranged, and redistributed in the low grounds of central Europe? To answer this question we must examine more closely the relation borne by the löss to the fluvio-glacial deposits and morainic accumulations. We note, in the first place, that in its horizontal distribution it follows closely that of the valley gravels of glacial times. Where the latter are well developed, the löss appears in full force; where they are wanting there is a like absence of löss. In all the valleys leading down from the Alps to the low grounds of middle Europe the loss puts in a prominent appearance. It obviously bears a close relation to the main lines of drainage, and may be said to be confined to valleys that head in formerly glaciated areas. So, again, in north Germany and southern Russia it spreads over all the low-lying tracts that lay in front of the vast mers de glace of glacial times. These facts alone, taken in connection with the occasional well-stratified character of the löss, the intercalation in it now and again of beds of sand, and the presence ever and anon of fresh-water shells, seem strongly suggestive of a fluviatile origin. And that such was really the origin of the materials of the löss will appear clear enough when we consider the conditions that obtained during a glacial epoch. (See Map A.)

While all northern and northwestern Europe were covered by an ice sheet, the mountains of middle Europe and the alpine lauds supported great glaciers, which in many cases deployed upon the low grounds. Vast bodies of water must then have escaped from the terminal front of the northern mer de glace, while the streams and rivers flowing from our mountain tracts must have greatly exceeded their present successors. With each recurring spring and summer wide areas in the low grounds would thus be subject to floods and inundations. Coining from regions where glacial grinding was being carried on upon a most extensive scale, it goes without saying that all these waters would be clouded with the fine flour of rocks. The enormous morainic accumulations formed underneath and in front of the alpine glaciers, and over the vast areas traversed by the Scandinavian mer de glace, bear emphatic testimony to the intensity of glacial erosion. In like manner the great terraces of gravel that stretch down the valleys in front of the alpine moraines and the broad sheets of similar deposits which extend outward from the glaciated tracts of northern Europe, are equally impressive witnesses to the vigor of the flooded glacial rivers. It is certain, however, that gravel, grit, and sand would not be the only materials carried forward by those rivers. As they reached the low-lying tracts their rate of flow would gradually diminish, and finer-grained materials—fine silt and loam—would eventually be deposited. When we consider the great volumes of water descending to the low grounds, we can not, indeed, escape from the conclusion that many wide areas in the plains during a glacial epoch must have been inundated, and in those slack waters and temporary lakes the finer-grained fluvio-glacial sediments would tend to accumulate. We must also bear in view the probability—I had almost said the certainty—of great derangements of the drainage having taken place in middle Europe. In winter, when the rivers of that region were frost bound, snow must frequently have drifted to great depths in the valleys, and the spring and summer thaws would often fail to remove these heaps. In this way the valleys might here and there become entirely filled with the blown and congealed snows of successive years, so as to compel the rivers in summer to rise in flood and to reach levels which they might otherwise have been unable to attain. We have positive proof, indeed, that such accumulations of drift snow actually did appear in extra-glacial regions, for some of them have persisted to the present day. The ice formations of the arctic coast lands, with their associated mammalian remains, certainly belong to the glacial period. They are simply the drifted snows, now converted into granular and massive ice, which accumulated in valleys and depressions outside of the glaciated regions. Protected under a covering of superficial detritus, alluvial matter, and peat, they have in those high latitudes persisted to the present day. Farther south, in central and western Europe, similar masses of congealed snow, as we have seen, appear to have accumulated, and may well have endured for some time alter glacial conditions bad passed away. In these temperate latitudes, however, they were bound ere long to melt and allow the overlying alluvial deposits to settle down in the manner already described.

There are thus various lines of evidence which lead to the conclusion that during a glacial epoch the lower reaches of all the great valleys opening out from glaciated regions, as well as large tracts of the wide plains extending in front of the northern mer de glace, would be more or less drowned in temporary lakes of turbid water, over the beds of which a line sediment of somewhat uniform character must have been deposited. And such is generally believed to be the origin of the materials of the löss. The loss, as we now have it, is a fluvio-glacial silt or loam, very largely reassorted and rearranged by the wind. Its history, therefore, is involved with that of the Ice age, and we must consequently turn our attention to the unquestioned deposits of that period, with a view to discover, if we can, at what particular stage of it the glacial silts were worked over by the wind, and tundra and steppe faunas successively occupied the low grounds of middle Europe.

Let us first, then, trace as briefly as may be the history of the glacial and interglacial deposits. Avoiding detail, we shall confine attention to the more salient features of the evidence and try to picture the succession of events from the beginning to the close of glacial times.

The facts upon which geologists base their conclusion that a vast ice sheet formerly covered much of northern and northwestern Europe, while great snow fields and glaciers existed not only in the Alps, but in many of the minor mountain ranges of central and even of southern Europe, may be very briefly summed up.

First, we have the evidence supplied by morainic accumulations of all kinds—bottom moraines or bowlder clays and terminal moraines. Second, we have the proofs of former glaciation afforded by striated rocks and roches moutonnées and by the crushed, broken, tumbled, and confused rock surfaces that occur so frequently underneath the bottom or ground moraines. Third, we have the presence of certain remarkable ridges of gravel and sand which appear to have been formed in tunnels under the ice, and of enormous sheets of similar materials which have been spread out by the waters escaping from the terminal front of the inland ice of northern Europe, while in all the great valleys leading down from the Alps and other glaciated mountains we see broad terraces of alluvial detritus which have been deposited by torrential streams and rivers. All those fluvio-glacial deposits, when followed from the low grounds into the regions occupied by moraines, are found to dovetail with the latter and are consequentl—of contemporaneous origin.

By mapping rock stripe and noting the general trend of the erratics which constitute so large a portion of the ground moraines we acquire a knowledge of the directions followed by the inland ice and the great glaciers. Not only so, but by tracing the horizontal and vertical distribution of glacial phenomena we have been able to show what regions were wholly ice covered, to measure the thickness attained by ice sheets and glaciers, and to estimate the angle of their surface slope. It is, in short, quite possible now to draw maps of Europe which shall give a fairly accurate presentment of the aspect presented by our continent in glacial times. On maps of a sufficiently large scale we can delineate not only the great inland ice of the north and northwest, but the snow fields and numerous glaciers of the Alps and other mountainous tracts, together with the areas covered by fluvio-glacial deposits.

So much for what we may call the physical evidence. But this is not all, for associated with the true glacial accumulations occur in many places beds charged with the remains of arctic-alpine plants and animals. The evidence of fossil-organic remains, therefore, fully supports the conclusions arrived at from a study of purely glacial phenomena. We know that arctic forms of life lived in our seas at the time of which I am speaking, and that the countries outside of the glaciated areas were then clothed and peopled by an arctic-alpine flora and fauna.

But, as if in contradiction of this evidence, certain other deposits charged with the remains of temperate and southern species of plants and animals appear intercalated among the true glacial accumulations. The study of these and of their relation to subjacent and overlying morainic and fluvio-glacial accumulations has led to the conclusion that the Glacial period was not one continuous period of arctic conditions, but a cycle or succession of alternating cold and genial epochs.

So far as we at present know, glacial conditions first supervened in late Tertiary times—in the so-called Pliocene period. In the earlier part of that period the European climate had been singularly genial. Warm seas, tenanted by many southern species of mollusks, washed the shores of the British area, while the land was clothed with a much more varied and abundant flora than we now possess. Great forests seem to have covered vast areas, occupying not only the plains and the river valleys, but extending far up the mountain slopes of such regions as France without much change of character. The same species, indeed, appear to have flourished equally well in Cantal and central Italy. Some of these had come down from early Tertiary times and were destined soon to become extinct; some, again, were special forms belonging to genera which in our day are exotic; others were species which have survived to the present in more southern and eastern regions, while yet others are still represented in Europe by identical or very closely allied species. Thus the flora of the Pliocene was connected both with the past and the present plant life of Europe, while at the same time it had relations with the floras of distant southern and eastern regions—with Florida, the Canary Islands, China, and Japan. All the evidence thus implies for early Pliocene times an equable and uniform climate, which permitted the intimate association in our continent of many plants which are now no longer able to exist at similar elevations or in one and the same latitude.

The mammalian life of Europe in early Pliocene times was in keeping with the flora. The deinotherium and mastodon still survived, and along with these were rhinoceroses, hippopotamuses, and elephants, and many cervine and bovine animals. Carnivores of extinct and still existing types and many monkeys were also present.

Such, then, was the character of the climate, and the aspect of the flora and fauna of Europe in preglacial times. The gradual approach of glacial conditions is evidenced by the fact that the percentage of northern and arctic shells in the upper Pliocene marine deposits increases from the lower to the higher members of the series. We note a gradual dying out of southern species and a gradual coming in of northern forms, until at last the beds are charged with the remains of a truly arctic marine fauna. We have no direct evidence as to the terrestrial conditions which obtained in Britain and Ireland at that time. The climate, however, could not have been genial and temperate as it is now. The presence of an arctic fauna in our seas shows that our shores were washed by currents coming from the north, and not as at present from the southwest. Reasoning from the analogy of to-day, therefore, we might infer that the climate of our area was probably not unlike that of Labrador.

The traces of the first glacial epoch are more clearly read in the deposits of the continent. An immense glacier at this time, fed from the uplands of Scandinavia, filled the basin of the Baltic. The bottom moraine of that great ice flow is seen in the low grounds of Scania, in southern Sweden, while its fluvio-glacial deposits have been detected at many places in north Germany. The alpine lands were contemporaneously covered with extensive snow fields, and large glaciers descended the deep mountain valleys, to deploy upon the Vorländer, in Switzerland, and south Germany. The terminal moraines of these glaciers have been mapped out, and the general conditions of the epoch have been so well ascertained that the position of the snow line at the time has been determined. It is believed to have been upon an average some 4,000 feet lower than now. While the valleys of the Alps were thus gorged with ice and the basin of the Baltic was occupied by an immense mer de glace, it is not probable that the higher parts of our islands could have escaped glaciation. We can hardly doubt that snow fields and glaciers must also have existed here. No trace of these, however, has been or is ever likely to be detected. Direct evidence of the kind, if it ever did obtain, has been obscured or destroyed by the action of the much greater glaciers and ice flows of later epochs.

In tracing the succeeding events in the geological history of Europe, I shall confine attention in the first place to the alpine lands, for it is in the low grounds at the base of those mountains that the relation of the loss to the glacial and fluvio-glacial deposits can be most clearly made out.

It has now been ascertained that glaciers have on three successive occasions filled the great mountain valleys of the Alps and descended to the low grounds. The earliest advance I have already described—this constitutes the first glacial epoch of Swiss geologists. It was followed by a long spell of genial conditions when the great glaciers melted away, and retired to the inner recesses of the mountains. Many relics of the flora of this genial epoch have been preserved. Thus in the valley of the Inn, near Innsbruck, certain deposits have yielded an assemblage of plants similar to that which we now meet with in the valleys of the mountain regions south of the Black Sea—most of the plants being existing species. The mean annual temperature of the regions in which that flora now flourishes is 57° to 65° F., while that of Innsbruck at present is only 47°. But in the genial epoch of which I speak, the flora in question flourished on the mountain slopes over-looking Innsbruck at elevations of 3,600 to 3,900 feet, where the mean annual temperature in our day does not exceed 40°. This is enough to show us that the climatic conditions of the alpine valleys must formerly have been considerably more genial than at present. From this and similar evidence in other alpine valleys we may safely infer that the retreat of the glaciers was the result of a great change of climate, and that during the first interglacial epoch the snow fields and glaciers must have retired to the highest ridges of the mountains.

The plant beds just referred to are not only underlaid, but overlaid by bottom or ground moraines, the overlying moraines belonging to the second glacial epoch. It was during this epoch that the glaciers of the Alps attained their greatest development—the snow line becoming depressed to 4,700 feet below its present level. The glaciers now pushed their way into the low grounds considerably beyond the limits reached by their predecessors in the first glacial epoch. That the second, like the first glacial epoch, was of long duration is shown by the amount of erosion effected by the ice flows and the enormous extent of their bottom and terminal moraines.

Overlying the ground moraines of that epoch we again come upon alluvial deposits in many places, which are crowded with the remains of a temperate flora—a flora resembling that of the low grounds of Switzerland and north Italy in our own days. It is obvious, therefore, that when such a flora flourished in the great valleys of the Alps the climate could not have been less genial than the present; the snow line must have again retreated to a higher level, and the névés and glaciers were probably not more extensive than they are now. This constitutes the second interglacial epoch of Swiss geologists. Ere long it was followed by a third general advance of the glaciers, which once more reached the low grounds at the base of the Alps, but did not flow so far as their predecessors of the preceding or second glacial epoch. The snow line of this third glacial epoch stood at an average level of about 4,400 feet below the present.

Each glacial epoch was necessarily marked by profound glacial erosion, and the consequent formation of massive sheets of ground moraine in the lower reaches of the great valleys, and of huge terminal moraines at or opposite their mouths. Enormous quantities of shingle and gravel were at the same time swept outward by the rivers escaping from the ice—each series of terminal moraines being thus closely associated with its separate and distinct set of fluviatile deposits. No difficulty is found in separating those successive accumulations of gravel. They form terraces lying one within the other at three successive levels. The highest rises upon an average 250 to 300 feet above the present rivers; the surface of the middle terrace is about 100 feet below the surface of the highest, and about the same distance above the level of the lowest terrace. Each terrace rests upon solid rock, and it is obvious, therefore, that the several epochs of gravel accumulation have been separated by epochs of active river erosion. This remarkable valley-within-valley formation is clearly the result of climatic changes. The highest terrace indicates the action of flooded rivers escaping from the glaciers of the first glacial epoch. These glaciers then disappeared or shrank into comparative insignificance, and an interglacial epoch of active valley erosion succeeded—the rivers cutting their way down for a hundred feet or more into the solid rocks. Next came the second glacial epoch, and the lowered valley bottom was again deeply covered with gravel. The glaciers of this stage then in their turn retired, and a second interglacial epoch supervened, when the rivers as before deepened their channels, working down through the older gravels and excavating the underlying rocks. Thereafter the third glacial epoch ensued, and a new series of gravels was deposited at a lower level than the preceding accumulation. Lastly, this third glacial epoch passed away and the rivers again trenched the fluvio-glacial gravels, the upper surface of which is now much above the reach of the greatest floods.

What relation, then, does the löss bear to the glacial and interglacial accumulations of the alpine lands? Fortunately to this question a definite reply can be given. It is dovetailed with the glacial deposits in such a manner as to show that its formation has taken place at successive epochs. Thus it occurs occupying an interglacial position between the accumulations of the first and second, and between those of the second and third glacial epochs. When we pass down the valley of the Rhine a similar succession is encountered. In the wide plain lying between the Vosges and the Black Forest, löss is met with on the same geological horizons, overlying the gravel terraces of the first and the second glacial epochs. Not only so, but even the youngest or lowest gravel terrace (that of the third glacial epoch) is in like manner sheeted in löss. The loss on these three separate horizons is for the most part wind blown, and exactly resembles that of middle Europe generally, showing the same structure and arrangement, and containing a similar assemblage of organic remains.

To what extent each of these "horizons" of löss may be represented in the low grounds of middle Europe we can not definitely say. But as the materials of the loss are for the most part of fluvio-glacial origin, it is obvious that such accumulations must have been formed during each successive advance of the alpine glaciers. As each glacial epoch passed away those accumulations were greatly modified by the wind, and drifted into the valleys that drain the Alps, where they were subsequently covered and to some extent preserved under the morainic and fluvio-glacial deposits of the succeeding epoch of glacial advance. It seems probable, therefore, that the wind-blown loss of the low grounds of middle Europe does not belong exclusively to any one particular stage of the glacial period. It is impossible, however, at present to divide it up into separate stages. But we may feel sure that if tundra and steppe faunas succeeded each other again and again in the valley of the Rhine, they could hardly fail to have done the same in the wide plains of middle Europe.

It will be remembered that at the Schweizersbild the deposits containing remains of tundra and steppe faunas rest immediately upon fluvio-glacial gravels. These gravels were laid down during the third glacial epoch. It is quite certain, therefore, that the faunas referred to must have entered Switzerland after the retreat of the glaciers from the low grounds. But how long an interval may have elapsed between the disappearance of the glaciers and the advent of the lemmings and their congeners we can not tell. All we know is that after the appearance of the tundra fauna in Switzerland the climate, at first cold and arctic, gradually became less extreme, so that in time a steppe fauna, and afterwards a forest fauna, succeeded. In other words, no perceptible hiatus separates the present from the conditions that obtained when the reindeer hunter vanished from the alpine lands. He was succeeded by Neolithic man, just as the latter was followed by the men who used bronze and iron implements and tools. So far as the evidence of the Schweizersbild rock shelter is concerned, we should infer that no great alternations of cold and genial epochs followed after the final retreat of the great glaciers of the third glacial epoch. But, as we shall see presently, the tale told by that interesting rock shelter is incomplete. Certain considerable climatic changes did take place after the third glacial epoch had passed away. The evidence of such change, however, though not wanting in the alpine lands, is much more clearly displayed in northwestern Europe. To the testimony yielded by the glacial and interglacial deposits of that region, therefore, we shall now direct attention.

It will be remembered that during the first glacial epoch a great Baltic glacier existed, and an arctic fauna lived in the North Sea. That epoch was succeeded by the first interglacial stage, when the southern part of the North Sea became dry land, and England was occupied by an abundant mammalian fauna—comprising hippopotamus, elephants, rhinoceros, horse, bison, boar, many kinds of deer, and a number of carnivores, including bears, hyena, saber-toothed tiger, wolf, fox, etc. The contemporaneous flora was temperate, resembling very much that which now exists in southeast England. In similar latitudes on the continent the same mammalian fauna flourished, while the flora was temperate, but suggestive of less strongly contrasted summers and winters than the present. A kind of insular climate, in short, seems to have characterized north Germany.

To this genial interglacial epoch succeeded the second and most extreme of all the glacial epochs. An enormous mer de glace then extended over all northern and northwestern Europe, from the British area in the west to the Urals in the east, and from Lapland in the north to the mountains of middle Europe in the south. (See Map B.)

When these extreme conditions eventually passed away, the second interglacial epoch supervened, characterised, as the earlier one had been, by a genial temperate climate, by the presence in England and the continent of the great pachyderms and their congeners, and by the appearance of Paleolithic man.

This second interglacial epoch was in its turn succeeded by a third advance of the Scandinavian "inland ice," which once more coalesced with the mer de glace of the British area. It did not, however, flow so far as its predecessor. Nevertheless, it reached the Valdai Hills in the east, the valley of the Elbe in the south, and covered all Scotland, the north of England, and the major portion of Ireland. This ice flow was most probably contemporaneous with the third advance of the great glaciers of the Alps. (See Map C.)

It is noteworthy that the löss in north Germany nowhere overlies the morainic accumulations of the third glacial epoch. It does, however, cover the marginal area of the ground formerly invaded by the second and greatest mer de glace. This clearly shows that the löss of north Germany must belong, in part at least, to the second interglacial epoch. The fact that it everywhere avoids the regions over which the third great ice sheet prevailed, does not, however, prove that tundra and steppe conditions did not supervene at a later date in middle Europe. The evidence supplied by the alpine lands, and the great valleys that drain those lands, is quite conclusive of the contrary. There is no doubt whatever that the Paleolithic reindeer hunters followed the chase in middle Europe long after the third great Scandinavian mer de glace had retired from the plains of north Germany. The geographical distribution of the wind-blown loss shows that steppe conditions were restricted to a broad belt of land in middle Europe. These conditions were rendered possible by the former greater extension of our continent into the Atlantic, when the major portion of the North Sea and the English Channel were dry land, and the British Islands formed part of the continental area.

Considerable climatic changes continued to take place after the passing of the third glacial epoch. These have left their traces in the alpine lands, but they are nowhere so clearly seen as in northern and northwestern Europe. Temperate conditions supervened in north Germany, the flora and fauna closely resembling those of the present. But eventually a relapse to glacial conditions followed, and from the Scandinavian snow fields another invasion of north Germany took place. Norway, Sweden, and Finland were now once more shrouded in ice, and a great Baltic glacier came into existence, the gigantic terminal moraines of which are met with in Denmark, Schleswig-Holstein, and Prussia. The Scottish Highlands and other mountainous parts of the British Islands at the same time nourished local ice sheets and large valley glaciers, which in many cases descended to the sea. The alpine lands in like manner witnessed a recrudescence of glaciation, large glaciers flowing into the great longitudinal valleys, but nowhere deploying as before upon the low grounds. It is to this stage, probably, that we should assign the tundra fauna of the Schweizersbild. (See Map D.)

The succession in that interesting rock shelter has shown that as the severity of the climate relaxed, steppe and forest faunas successively followed the disappearance of the tundra forms. The climate of Europe generally became temperate, and immense forests overspread wide regions. It was during the approach of these conditions, as we have seen, that Paleolithic man seems finally to have vanished and the Neolithic races to have made their earliest appearance in Europe. The British Islands at this time formed part of the continent and the Baltic existed as a great fresh-water lake. The lower buried forests of our peat bogs are among the conspicuous remains of this stage. Eventually, however, submergence ensued, the British Islands were severed from the continent, and the sea again invaded the Baltic basin. It is notable that the character of the marine fauna which at this stage lived off the coasts of Scandinavia and Britain is indicative of more genial conditions than now obtain. The climate, however, gradually became colder, the vertical and horizontal range of the forests was restricted, and snow fields again appeared among the higher mountains of our islands. In Scotland glaciers here and there came down to the sea, and dropped their moraines upon the beaches then forming; the large majority, however, terminated inland. At that time the snow line in north Britain ranged between 2,000 and 2,600 feet. Similarly, in Norway and in the Alps an advance of glaciers took place—the snow line in southern Norway being about 2,400 feet, while in the alpine lands it seems to have averaged 7,500 feet, or some 1,600 feet lower than the present.

Later climatic oscillations followed, but ou a decidedly reduced scale. The effect of these was, naturally enough, most marked in northwestern Europe, decreasing gradually southward, and doubtless eventually fading away in the lower latitudes of the continent. It is not necessary for my present purpose to do more than briefly indicate the general character of these later changes so far as they affected our own area.

The local glaciers of the British mountains, some of which, as I have said, actually entered the sea, at last began to retreat. The climate became more genial, and so once more favored the growth of forests, which in many places began to overspread the now dry peat bogs, beneath which the trees of the earlier forest epoch lay entombed. Eventually, however, colder and more humid conditions returned, and small glaciers appeared in a few places among the loftiest heights of the Scottish Highlands. The position of the moraines of these glaciers indicates a height of 3,500 feet for the snow line. The forests now, as before, began to decay in many places, and the bog moss and its allies again extended in all directions, and so, eventually, a second forest bed became entombed in growing peat. It is needless to say that the evidence of these later changes is not restricted to Scotland. The bogs of the two sister countries, and of the corresponding latitudes on the continent, present us with precisely the same phenomena.

The present decayed aspect of the bogs in many places where they formerly flourished, and the fact that certain plants and groups of plants are once more beginning to invade such wastes, shows that we are now living under somewhat milder and less humid conditions.

Although these later climatic oscillations certainly affected the distribution of plants and animals to some extent in northern and north- western Europe, yet the changes brought about were insignificant as compared with those which characterized the alternations of preceding glacial and interglacial epochs. The earlier cold and genial stages were strongly contrasted, and marked by great migrations of flora and fauna. But, as the strange cycle drew to a close, the contrast between glacial and interglacial phases became less and less pronounced and gradually faded away into the present. The steppe fauna vanished from middle Europe during the fourth interglacial epoch, and it never returned. The climatic oscillations that followed were on too small a scale to induce great migrations, and thus the succeeding forest fauna retained its place. Hence in such a section as that seen in the rock shelter of Schweizersbild, we find no recognizable evidence of the climatic changes to which the buried forests and peat bogs and the small local moraines of northern and northwestern Europe bear testimony. It is thus only by correlating and comparing the evidence over the widest area that we are able to get the story completed.

In fine, we have seen that tundras and steppes appeared at successive epochs in prehistoric Europe. The former were contemporaneous with the great ice sheets and glaciers, while the later came into existence when glacial conditions were passing away. The tundra and steppe conditions of our continent belong, in short, to that remarkable cycle of climatic and geographical changes known as the Ice age or glacial period. Paleolithic man undoubtedly lived through both phases, for his relics and remains are found associated alike with the arctic lemmings and the succeeding steppe animals. Whether the reindeer hunter of middle Europe ever came into contact there with the Neolithic man we can not tell. Were we to trust to negative evidence we should say he never did. But negative evidence can not be trusted. It is quite possible that the two races may have met and even commingled, but of this no proof is forthcoming. The strong hiatus that separates the Old Stone and the New Stone epochs in western and northwestern Europe has not yet been bridged over in middle and southern Europe. When last we see Paleolithic man he is hunting the reindeer and the mammoth in the Danubian steppes. His Neolithic successor seems not to have appeared in middle Europe before steppe conditions had passed away and a forest flora and fauna had become dominant.

	PLATE 1

THE TUNDRAS AND STEPPES OF PREHISTORIC EUROPE
(REPRODUCED FROM SCOTTISH GEOGRAPHICAL MAGAZINE)

1. A lecture delivered before the Royal Dublin Society, March 9, 1898. Those interested in the subject of this lecture will find it fully discussed by Professor Nehring in his work Ueber Tundren und Steppen. See also, for further information and for references to other authorities, The Great Ice Age, Chapter XXXVIII. Reprinted from The Scottish Geographical Magazine, Vol. XIV, Nos. 6 and 7, June and July, 1898, pp. 281–357.
2. A lecture delivered before the Royal Dublin Society, March 11, 1898.