Popular Science Monthly/Volume 4/April 1874/The Age of Ice
|THE AGE OF ICE.|||
TILL within a comparatively recent time, geologists regarded the climate of the prehistoric periods as tropical or warm temperate. Those who first sought to explain the presence of certain scratches upon ledges, by the action of moving ice in continental masses scouring the surface, were met by ridicule and skepticism. The writer has now before him a caricature devised to illustrate the notions of the literary world upon this subject thirty years since in England. The excellent Dr. Buckland appears clad in furs, such as are required in Greenland, with a map of ancient glaciers under his arm, showing markings made on the rocks 33,333 years ago. On one side is represented a bridge with a scratched paving-stone at the entrance, and an inscription like this: "Scratches made day before yesterday by a cart-wheel passing over Waterloo Bridge." It is said that the learned doctor was greatly amused by the sketch, and sent copies of it to all his scientific friends. The one before us bears his autograph.
But within the last three decades numerous observers have carried out the suggestions of the earlier geologists to a very extensive application. Forbes and Agassiz explored the glaciers of Switzerland in order to learn the laws of ice-motion; Lyell, Murchison, Ramsay, and others, have ransacked the fields of Great Britain in search of facts from which to generalize; and, in our country, Hitchcock, Mather, Whittlesey, Newberry, Dana, and a score of younger men, have made the investigation of the drift period a matter of enthusiasm. The existence of an immense era when all of Northern America and Europe was enveloped by enormous thicknesses of solid ice, crunching fragments of rocks beneath its massive tread, and transporting square miles of moraine rubbish upon its back, is now universally accepted. Some have gone so far as to believe that the entire globe was encircled in these arctic fetters—as illustrated by the Brazilian researches of the lamented Agassiz.
One of the latest and most extensive works relating to the Ice period is given to the world by James Geikie, of the Royal Geological Survey of Scotland. The subject is treated of with special reference to the phenomena exhibited in the north of Europe. He subdivides the Ice age as follows:
1. Preglacial Period.—This is seen best in the "Norwich crag," where remains of the elephant and mastodon are found in peat-beds, and these are indications of approaching cold.
2. First Cycle of Cold.—This exhibits intense glacial conditions, with great confluent glaciers; intermediate mild and warm periods; arctic and southern mammalia visit Britain alternately, according as climatal conditions become suited to their needs. This is followed by an arctic climate with the mountains covered by snow and ice, the glaciers ceasing to be confluent. The era terminates with local glaciers. The deposits laid down are chiefly the "till" and "bowlder-clay," with a few stratified sands.
3. Last Interglacial Period.—In this Britain is at first insular, with cold climate; next continental, with climate changing from cold to temperate and genial, and again to temperate. In early stages of the continental condition, the arctic mammalia invade Britain. Subsequently these disappear, and are succeeded by the hippopotamus, etc.; afterward submergence ensues, and the islands are again insulated, perhaps before the climate became again suited to arctic mammalia. At the close of this period the land sinks, reaching the depth of 2,000 feet below its present level in Wales. The deposits of this era are cave-accumulations, river-gravels, and high level beaches. The human implements found with the extinct mammalia are of stone, and of the rudest construction.
4. Last Glacial Period.—This was a time of floating ice, comparable with the conditions imagined by the earlier writers for the whole drift period. The climate was arctic, icebergs floated over most of the land, enormous blocks of stone got stranded upon the hill-tops, moraines clogged up valleys, and toward the termination of the period local glaciers manifested the final effort of the ice to gain the mastery. Remains of boreal shells and the mammoth occur for the last time in the frozen sands and ancient beaches.
5. Preglacial Period.—The land has regained its present level, terraces are formed by immense rivers, arctic forms of life have disappeared, and the era of bronze and iron implements shows what progress man has made in the arts.Till.—Throughout the length and breadth of Scotland occur scattered heaps and ragged sheets of sand, gravel, and coarse débris, together with wide-spread deposits of clay largely obscuring the solid ledges. In the Highland and upland districts these deposits seem to be
largely restricted to the valleys, while in the Lowlands they spread in broad sheets, continuous over wide tracts. The very bottom earth is a strong clay or till, much like our hard-pan, which is therefore older than any of the overlying deposits. Their relations are shown in the accompanying figure.
This till is so tough that engineers would much rather excavate the most obdurate rocks than attempt to remove it from their path. Hard rocks are more or less easily assailable with gunpowder, and the numerous joints and fissures by which they are traversed enable the workmen to wedge them out often in considerable lumps. But till has neither crack nor joint; it will not blast, and to pick it to pieces is a
very slow and laborious process. Should streaks of sand penetrate it, water will readily soak through, and large masses will then run or collapse, as soon as an opening is made into it.
The percentage of stones present is variable. They are most common in the hilly districts, while in the lowland region the clay may predominate. Most of them show markings all over. They vary in size from grains to blocks several feet or even yards in diameter. Their shape is peculiar. They are neither round nor oval like the pebbles in river-gravel, or the shingle of the sea-shore; nor are they sharply angular like newly-fallen débris at the base of a cliff; but seem to be like the latter in general shape with the sharp corners and edges smoothed away. They are to the geologist what hieroglyphics are to the Egyptologist—the silent but impressive records of an age long passed away.
In narrow valleys the till often accumulates in such amount as to cover the solid floor many yards in depth. In such cases, the surface may be level, and, in the subsequent periods, the streams have made excavations in the mass, leaving the till in the terrace-form. Its unstratified character will be determined by examining the earth along
the sides of the escarpment, as, superficially, it is difficult to distinguish the material from the terraces of later age.
When the till is removed from the underlying rocks, their upper surface almost invariably shows a smoothed and often highly-polished appearance, and the whole pavement is marked with those peculiar scratches or striæ that form so characteristic a feature of the embedded stones. The extent to which the polishing is carried depends very much upon the nature of the rock. As the best-preserved stones of the bowlder-clay consist of close-grained limestone and clay iron-stone, so the same materials in the ledge-condition preserve most perfectly the fine lines of striation. The soft sandstones and highly-jointed rocks are much less finely marked, and often show a broken and shattered surface.
Greenland.—To understand the appearance of Northern Europe in the Ice period, we may consider the features presented by a similar ice-covered country in modern times, and no country will better illustrate this phase of geological condition than Greenland. This island is almost continental in its dimensions, containing not less than 750,000 square miles, and is all a bleak wilderness of ice and snow, save a little strip extending to 74° north latitude along the western shore.
The coasts are deeply indented with numerous bays and fiords or firths, which, when traced inland, are almost invariably found to terminate against glaciers. Thick ice frequently appears, too, crowning the exposed sea-cliffs, from the edges of which it droops in thick, tongue-like, and stalactitic projections, until its own weight forces it to break away and topple down the precipices into the sea. The whole interior seems to be buried beneath a great depth of snow and ice, which loads up the valleys and wraps over the hills. The scene opening to view in the interior is desolate in the extreme—nothing but one dead, dreary expanse of white, so far as the eye can reach no living creature frequents this wilderness—neither bird, beast, nor insect. The silence, deep as death, is broken only when the roaring storm arises to sweep before it the pitiless, blinding snow.
This represents perfectly the state of the northern part of our continent in the Ice age. We have a slight inkling of what it must have been universally, from the heroic messages sent down in the winter from the meteorological observatory stationed upon the summit of Mount Washington.
Some of the Greenland glaciers attain a vast size. Dr. Kane reports the great Humboldt glacier (see Fig. 1) as sixty miles wide at its termination. Its seaward face rises abruptly from the level of the crater to a height of 300 feet, but it is not known how deep it may extend under the sea. Another important ice-stream is the Glacier of Eisblink, on the northwest part of the island. It projects seaward so as to form a promontory thirteen miles in length. It comes from an unknown distance in the interior, and plunges deeply into the sea.
Since ice is lighter than water, whenever a glacier enters the sea the dense salt-water tends to buoy it up. The great tenacity of the frozen mass enables it to resist the pressure for a time. By-and-by, however, as the ice reaches deeper water, its cohesion is overcome, and large segments are forced from its terminal part, and floated up from the bed of the sea, to sail away as icebergs. The glacier evidently crops under the water to considerable depths, or, so long as the force of cohesion is able to resist the tendency of the salt-water to press it upward. The annexed diagram will show how the ice pushes down into the sea, carrying morainic materials at its base, which accumulate at m; t b may show the origin of the block i, which is now going to sea as a buoy. In many cases the icebergs must carry with them stones frozen on the under side, as well as blocks perched on their backs. Dr. Kane speaks of ice-rafts, floating many miles out to sea—tables 200 feet long covered with large angular blocks and bowlders.
Though Greenland is said to be inhabited only upon the south and west coast, there is a record of an early settlement upon the side toward Iceland, with which there has been no communication for 400 years. The colony was planted about 1000 a. d., which flourished, and maintained intercourse with its mother-country till the beginning of the fifteenth century. Since that time, owing to the setting in of the arctic current, and the consequent gradual increase of ice upon the coast, the colony became inaccessible, and the records of it disappear from history. At various intervals between, 1579, 1751, etc., down to our own time, the intrepid Danes have striven in vain to reopen communication with their lost colony. This emerald coast, with valleys well stocked with reindeer and verdant glades, is now shut in by the pitiless ice-pack, and the fate of its inhabitants ought to excite the interest of the world. It would be very interesting to be informed of the condition of this colony: whether the increasing cold has enlarged the glaciers so as to push the dwellings out to sea, or whether the habitations are still standing, and a population has sprung up who know of the outside world only by tradition.
Lake-Basins.—A strong argument for the former existence of glaciers over the northern regions comes from the excavation of basins from the solid rock for the reception of lakes. The country most traversed by the ice agency abounds in these rock-hollows. It is very evident that the glacier is the only agency which can well be called upon to explain these phenomena. Running water excavates only on a descending plane. Sea-water acts upon its level, while the glacier requires only a pressure from behind to enable it to ascend mountains. The upward movement of the ice is shown by the striæ to have been exceedingly common.The glacier grinds hardest where the steeper slope is exchanged for a less inclination of its rocky bed; the tendency of this action is to
increase the length of the greater in elevation, or, in other words, to scoop out material from the ledges, in a part of the course, and, after the strength of the graver has exhausted itself, the ice will move up a slope with little energy. This process will excavate hollows that may be filled with water in later times. Such are the basins of the great American lakes. The dimensions of the lakes are areally proportionate to the extent of the drainage-system in which they occur.
Bowlder-Clay.—There is a distinction to be drawn, in Scotland, between the "till" and "bowlder-clay." The two deposits pass into each other on the Highlands, and Mr. Geikie proposes to limit the latter to the maritime districts. The bowlders of the clay are more rough and angular than those found in the till. The annexed section shows where the two deposits come into juxtaposition. This clay has not been met with more than 260 feet in vertical height above the sea. It contains an abundance of shells of Arctic mollusca. Possibly it is the "Champlain clay" of America.
Antarctic Ice-Sheet.—From a study of the ice of the Antarctic Continent, it is possible to understand the origin of icebergs, and the transportation of large blocks of stone, in "erratics." The water is deep, and thus buoys, of enormous size, may float northerly for hundreds of miles.Sir J. C. Ross attained the highest southern latitude on record, but found all his attempts to penetrate farther frustrated by a precipitous wall of ice, frequently 180 feet in height. For 450 miles he found this cliff unbroken by a single inlet. While coasting along this barrier his ships were often in danger from stupendous icebergs and thick pack-ice, extending in masses too compact to be penetrated. At one point the ice descended sufficiently low to allow Ross to look down upon it from the masthead. The upper surface appeared to be a smooth plain, shining like frosted silver, and stretching away as far as eye could reach into the illimitable distance. In principle, the sheet is the same with that figured in the north, but more extensive. Like this must have been some portions of the glacial sheet in Scotland, when the land was mantled in ice-covering, filling up the intervening straits and channels of the sea, and terminating far out in the Atlantic Ocean, in vertical cliff of blue ice.
Erratics.—These are of all shapes and sizes, occasionally reaching colossal proportions, and containing many hundred feet. Some are rounded, others are angular, and not a few exhibit marks of scarification. They may rest on base-rock, and, if carefully poised, may be made to oscillate by the form of the land, or these large blocks may appear on the till, angular débris, and hills of gravel. As a general rule, they prove to have been carried from higher to lower levels in Scotland, though many exceptions are recorded. There is one at the height of 1,020 feet on the Pentland Hills, which may have traveled westerly as much as eighty miles. It probably passed from one mountain across a wide valley before attaining its final resting-place. This is not so striking as the blocks lying nearly over the recently-completed Hoosic Tunnel, in Western Massachusetts, one of which weighs 510 tons, and has been transported from Oak Hill across a valley 1,300 feet deep. It has hundreds of lusty comrades, scattered in a southeasterly course for thirty miles.
Sometimes a large block is revealed by the washing away of the till around it. Those on the surface of gravel may have been carried by floating ice. To such blocks it is not easy to assign limits of the distance traveled, since icebergs may float for thousands of miles without melting.
Origin of the Cold Climate.—The question of the cause of the glacial cold has been discussed warmly for a long time. The opinion seems to be gaining ground that purely geological causes are not sufficient to account for the magnitude of the glacial distribution. The precession of the equinoxes, changing the times of the seasons, and the eccentricity of the earth's path around the sun, lengthening the winters and increasing precipitation of moisture, when combined with certain changes in the courses of ocean-currents, and some elevation of land in the north, may have together been instrumental in bringing around a period of intense cold. If it be possible to use the orbital changes as a guide to a chronological date for this term of cold, we can say it began about 240,000 years since, and continued for 160,000 years, terminating 80,000 years before a. d. 1260. The cold would have culminated about 30,000 years after its beginning.Granting such figures, we can understand that the glacial must have been the dark age in the earth's history—a terrible blight upon the flourishing faunas and floras existing in tertiary times in northern latitudes. The presence of warm temperate plants in Greenland has always excited interest, even to the proposal of very wild theories to account for the genial climate there of preglacial days. It may be that the American Sequoia traveled across the bridge anciently connecting Greenland with Iceland and Scotland, and that the renowned cedars of Lebanon are the cousins of their famed relations in California; but the connection has been severed by the ruthless ice-flow, and is not likely to be reestablished, unless our sun shall carry his
system of planets through a much warmer region than the space now encircling us.
Centres of Dispersion.—All existing glaciers flow from higher to lower levels as a rule—the only exception being that already stated, when the ice may be forced up-hill for a short distance. This may be well exemplified in the Alpine glaciers of the present day. These streams of ice all flow from the summits and axes of particular mountains along the valleys, and spread over the neighboring plains. The action is radial—proceeding from a central point or line outward.
The geologists have concluded that most of the ancient ice-movements in Northern Europe have been from centres of dispersion, like those in Switzerland. Examples are numerous. One of the most interesting is exhibited in Switzerland. The traveler finds there two prominent centres of glacial radiation—the Bernese and the Mont Blanc regions. Glaciers now flow westerly into the great valley of Switzerland and toward the Rhone from the former, and in the latter group the streams discharge upon the Italian plains on the south, and toward the vale of Chamouni on the north. A careful study of the vale of Chamouni shows that ice once filled it to the brim, for the embossed rocks carry striæ even to the height of 5,000 feet. Search for the sources of bowlders proves that large blocks on the southern flanks of the Jura Mountains must have been derived from Mont Blanc, sixty or eighty miles distant. Instead of passing down the Arne at Chamouni, the blocks proceeded northerly toward the Rhone, and thus across the great valley of Switzerland to the Jura. The magnitude of this ancient action equals much of the wonderful glacial phenomena of other districts in Europe, though hardly equal to what may be seen on this continent. But, being satisfied of the former enormous extension of the Alpine glaciers from examination of the striations and the dispersion of blocks, it is easy to generalize and refer similar phenomena in other countries, whose glaciers are extinct, to the same mighty cause.
In Scotland there may have been a centre of dispersion for glaciers from Ben Nevis, another in the south part of the province. In England, one in the Cumberland region; in Wales, one from Mount Snowdon. It is easy to discover the evidence of radial dispersion.
A combination of the glacial and iceberg agencies may be discerned in a map in Mr. Geikie's work, showing the courses of the striæ marked upon the rocks of Scandinavia. They diverge from the central water-shed between Norway and Sweden—part pushing toward. Iceland and Scotland, and part directed toward Lapland and the Baltic Sea. The distribution of the bowlders corresponds with these marks. Furthermore, these ice-masses seem to have come in contact with the water of the Baltic, and part have floated over Germany till high land obstructed farther movement, and a part may have been caught by the outflowing Baltic current, carried over the North Sea to the south part of England, and perhaps Iceland. At least, bowlders of Scandinavian origin are common in these regions, and have probably migrated in the way described. On the east shore of Scotland they are plenty; but, between these and those south of the Thames, none have been found, which fact has given rise to the theory of dispersion by means of icebergs through the Baltic.In years past the prominent topic of discussion in scientific associations has been the character of the ice-movements in the Glacier period. One school has stoutly defended icebergs as the active agent, the other has vigorously insisted upon land glaciation. The example before us seems to require both these agents to account for all the phenomena of this period. Both classes seem to be right, though neither can explain all the facts. Nature's domain is so vast that human intellects do not seem to be capable of grasping the whole truth at once. We are like the mariners who seek to penetrate to the north-pole. They have penetrated a little way beyond Spitzbergen—they have gone nearer the goal through the straits west of Greenland, and have made great exertions in some other quarters. Each party has its theory of the character of the unknown region, as derived from a partial survey. By-and-by the whole of this area will be known, and it
will then appear that each theory had its element of truth. In like manner we are endeavoring to attain to the proper conception of the condition of the earth in the Age of Ice. The whole truth has not yet been discovered. When fully revealed, it will appear far more magnificent and glorious than has now been surmised.
- The Great Ice Age, and its Relation to the Antiquity of Man. By James Geikie, F.R.S.E., F.G.S. 575 pages. D. Appleton & Co. vol. iv. —41
- Geological Magazine, vol. x., p. 541.