Popular Science Monthly/Volume 12/January 1878/The Ice Age I
|THE ICE AGE.|
AT the end of that long course of geological ages, from the Archæan to the Tertiary, which built up the solid portions of the earth in their present configuration, geologists now universally recognize, in the evidence before them, the presence of a remarkable and stupendous period—a period so startling that it might justly be accepted with hesitation; were not the conception unavoidable before a series of facts as extraordinary as itself, and which, partaking of its astonishing character, are explained upon no simpler hypothesis. This era is known as the Glacial. It was an era which has left its traces in unmistakable monuments over the surface of either hemisphere, and written its history in no less explicit characters upon their rocks. It was an epoch of arctic rigidity, when the temperature of the earth had become so lowered that the cold regions of either pole alternately were permitted to extend their previously contracted circles over the temperate latitudes, and to envelop with a universal and prodigious mantle of ice the lands which once, beneath milder suns, had been the home of an abundant and tropical vegetation. The skirts of that glacial sea which perennially spreads its icy and resplendent surface over polar lands had then, by a favorable conjunction of solar and terrestrial influences, been expanded so widely, that to within the latitude of 39° north its frigid folds hid the surface of the earth, while below the equator a similar period seems to have left scarcely less visible traces amid the forests and pampas of South America. The evidence which has established the actual presence of these arctic conditions over a great portion of our earth is complete and irrefragable, and, aided by the contemporaneous study of Alpine glaciers and the Greenland icebergs, we can draw conclusions as to the nature and the succession of events which these conditions occasioned.
It was Agassiz who first insisted, perhaps almost with trepidation, that Central Europe, England, Scotland, and Ireland, had been buried beneath thousands of feet of solid ice; that from the mountain-tops of Scandinavia, the Grampians of Scotland, the Lake Hills of England, and the summits of the Alps, had proceeded vast rivers of ice whose confluent seas had swept over Europe, and beneath their resistless, ceaseless, and perpetual advance, grooved it with valleys, channeled the courses of its rivers, engraved its rocks, scooped out its lakes, and scattered their burden of débris far and wide over its plains. The conception was a bold, almost a terrifying one; and, because the actual history and nature of glaciers was so little known, it was regarded with aversion and spoken of with contempt. Agassiz had laboriously studied the glaciers of the Alps, he had conned the lesson they taught with eager apprehension of its great significance, and he knew so well every characteristic of their work that he instantly recognized abroad the same indelible evidence of their past presence.
Venetz, Rendu, and Charpentier, had preceded him in glacial study, and had insisted upon an extension of the Alpine glaciers far beyond their present beds in past ages, but had not realized the immense utility of these views in explaining the glaciated surfaces of Europe. Forbes, Hopkins, and Tyndall, succeeded him in the investigation of glacial physics, and by their close scrutiny into the constitution of ice, and the laws of ice-making and glacial motion, fairly established a new department of physical science, and added confirmation to the views of Agassiz.
Now, let us examine some of these singular and hitherto inexplicable records, which elicited Agassiz's theory, and which, long before they were harmonized by that assumption, had been attentively examined by geologists and explained upon other grounds. Furthermore, we will review them without reference to the theory of glacial action, and only subsequently compare them with the effects now being produced wherever glaciers and icebergs are at work.
The rocks as they lie in place, the flanks and summits of mountains to heights of 5,000 and 10,000 feet, and the surfaces of outcropping masses over immense areas of the world, are all gauged in long, straight channels, sometimes a foot deep, sometimes eight feet deep, with widths from two to three feet. These grooves, of all dimensions, pass over the rock in groups like mouldings, and the rocks they occur upon are polished and oftentimes lustrous. The channels diminish in size to the faintest striæ, which, like sharp scratches, cover the surface, running along at times in parallel series, or diverging in different directions, as though the great primitive plane had varied its course over them, scouring with exquisite fineness.
These lines and runnels score the rocks over the Northern United States and Canada, throughout Europe, in Asia, and over the shores of South America. We discover almost instantly that in the same region they have the same direction; that they seem, as it were, to stream with us from the north; and that, wherever other scores contravene this, these secondary markings are themselves harmonious, indicating some subsequent action upon the rock, in character similar to the first, though varying in its motion, and probably restricted in its extent and importance. Thus the scores upon the rocks of New England point northwest and southeast, and only local derangements disturb this prevalent direction. The easting increases as we progress to the ocean, reaching its maximum in Maine and the borders of Canada; while, as we retire from the margin of the States, we observe that the scratches and grooves acquire a north-and-south direction, becoming nearly meridional over New York, and then slowly swing round to the west, until in Ohio, Indiana, Illinois, Missouri, and the western limits of the continent, they lie pointing northeast and southwest. Thus they assume a rudely-outlined radiation from the highlands of Canada, and stretch out from an hypothetical centre there like the multiplied spokes of a great wheel. In Switzerland they sweep down and out from the central ranges of the Alps in all directions, and, while locally uniform, they converge from the south, and east, and north, and west, toward the lofty slopes and pinnacles of this congeries of mountains. Over West Russia and Northern Europe, where the markings are discovered, they indicate the Scandinavian mountains to have been the seat of whatever disturbance or agency has, at a distant period, fluted and engraved the continent; similarly, as the rocks lie related to the Highlands of Scotland, the Lake Hills of England, or the mountains of Wales, the striæ impressed upon them extend toward every point of the compass. They stream north and south from the summits of the Pyrenees, from the peaks of the Caucasus, and down the valleys of the Himalayas. It must be understood, however, that these conclusions are based upon an average of the bearings of the grooves in each instance, and that these are infinitely varied by the construction and irregularity of the land.
Thus over greater portions of the world we find the rocks furrowed, polished, and striated, in long, frequently deep and rectilinear grooves, which lie in groups and series identical in direction, and pointing to associated highlands, or distant continental mountain ranges, as the source of whatever strange and inexorable instrumentalities have produced them. Over New York Island the gneissoid and granitic rocks, where they raise their tilted strata and broken shoulders above the ground, are scored frequently with deep and sinuous channels. In Central Park, along Fifty-ninth Street, up the west side, the contorted and twisted humps of gneiss are moulded in this way. Sometimes, where a rupture exists, and one part of an outcrop has fallen below the other, the grooves are continued on the lower half; frequently the lines are crowded together like rulings on a page, and again the groove is of irregular depth, its floor rising and falling as though hitches had occurred when it was first planed, the great chisel meeting resistance, or being thrown up at points along its path. In the White Mountains the sides of the mountains, the valleys, the top of Mount Washington, at 5,000 feet above the sea, are all cut with these strange furrows, the rocks polished, and the whole country bearing these evidences of past erosion wherever the naked rock meets the eye. Over Maine the same phenomena present themselves in endless succession, the grooves striping the country, and losing themselves in the sea along the coast, while they corrugate the borders of the innumerable bays, and the walls of the deep fiords that indent the shores. These furrows can be traced for miles across the country, cutting the three ranges that lie between Bangor and the sea almost at right angles, traversing these highlands as though they were level surfaces, dipping beneath the sea, and reappearing upon the sides of Mount Desert, to be again lost in the waters of the Atlantic. Unquestionably, over that sea-floor, could we follow their tracks, the same furrows continue to the verge of the continent which lies miles out to seaward, when the steep edge of the land falls precipitately to the true bottom of the ocean. Over the West, throughout Canada, and upon the ancient rocks of the Great Lakes, these evidences of past erosion exist upon an enormous scale. As a rule, these striæ indicate a planing surface advancing from the north, and, though a second series may occur, as upon the islands of Lake Erie, from east to west, whose furrows obliterate the first inscription, such phenomena are local merely, and infrequent. Again, upon the Sierras, the tops and declivities of the ranges are scored and engraved with the indelible signatures of past erosions, and the rocks of the barren wastes of British America are. signalized in the same manner. So much for striæ: we perceive their universal presence, and their marked reference to the north, or elevated regions which dominate over level plains.
The second feature of this epoch, designated by common consent the Drift, is a series of surprising facts, evincing, throughout all this deeply-scored and paneled country, the past presence of extraordinary transporting agencies. We find rocks of enormous size, in some instances weighing 3,000 tons, planted in fields and lowlands, or strewed over hills and moors, where no rocks lie in place, sunken in the soil where the lithology of the district is entirely distinct, while that of the monoliths themselves is identical with rock many miles northward. These gigantic bowlders, Titanic mementos of the past, are scattered over Central Europe, over Germany, Holland, and Russia, are identical in character, and can have no nearer origin than in the mountains of Scandinavia. Some of these blocks of stone are of incredible dimensions, and are accompanied by innumerable smaller ones that lie over these districts as if flung in sport by some pre-Adamite Antæus. They have served the most useful purposes in the flat countries through which they are found, being used for buildings of every description, and their smallest associates have helped to pave the highways between Hamburg, Magdeburg, and Breslau. Accredited in ruder times to the malevolent agency of man's spiritual foes, they were called devil-stones; but Science, recognizing their distant origin, has named them erratics, and the Germans, more picturesquely, wanderers. Not only are they found upon level and loamy lands, utterly unaccountable except by the assumptions of transportation, but they are also discovered capping the cliffs of mountain chains, hanging by the side of depths, over which they must have been carried, and into which, by the Nemesis of destiny, they are now doomed to fall. The Jura Mountains, north of the great valley of Switzerland, and opposite the western or Bernese Alps, along the frontier of France, are thus studded with these bowlders, some of them containing 50,000 to 60,000 cubic feet of stone. These have come from the Alps; they are crystalline rocks, gneiss, and granite, and they lie upon ridges of limestone. They are virtually nothing less than dislocated fragments of those abraded and decreasing hills perched upon the Jura cliffs. Prof. Guyot has placed, beyond all doubt, their home upon the summits and sides of the Swiss Alps, and shown that they have attained their present eminence by a positive carriage from these original localities. This position has indeed been made impregnable by a protracted and laborious survey of innumerable "wanderers" found upon the Juras, whose lithological character identified them with the Alpine formation, while it served to trace the probable path of their transmission. These blocks have been found at elevations ranging from 2,000 to 3,500 feet above the sea, and in Carinthia similar erratics have been described at great elevations, proceeding from an opposite quarter of the Alps.
In North America, and especially throughout the Northern States, the bowlders are numerous, often of great size, and indicating transits of many miles. Over the Eastern, Middle, and Northwestern States, bowlders, that have emigrated from distant points to the northward, occur in such abundance that they may almost anywhere be found if the inquirer will only examine the country he passes over. Upon Mount Katahdin, in the Moosehead region of Maine, stones can be seen, lying over 4,000 feet above the sea, fossiliferous in their nature and coming from northern sites; while toward Mount Desert, masses, some forty to fifty feet in height, are sprinkled everywhere, and, as in the case of the Dedham granite distributed to the south, invariably show northern origin. In Berkshire County, Massachusetts, these traveled rocks lie in long alignments, passing over the Lenox Hills, and extending in a generally southeasterly direction for fifteen or twenty miles, and have been filched from the Canaan and Richmond Hills across the line in New York, being of chloritic slate, with angular specimens of limestone intermixed. Some granites from Vermont, on the west of the Green Mountains, have been lifted over these barriers and transferred to the southern margins of Massachusetts; while in Vermont a bowlder weighing over 3,400 tons, and known as the Green Mountain Giant, has been drifted from the Green Mountains easterly across the valley of the Deerfield River, and planted 500 feet above that stream. In Michigan, near the Menomonee River, a field upon the northern slope of a mountain is densely covered with bowlders, so that a mile can be traversed without once touching the ground. Again, huge nuggets of copper, torn from the immense deposits of native copper at Keweenaw Point, Portage Lake, and the Ontonagon district, on the southern shore of Lake Superior, are found widely disseminated to the south of these localities in Michigan, Wisconsin, Ohio, and Minnesota, a few of which have weighed 300,800, and one 3,000 pounds. From the sides of the White Mountains fragments of rock have been carried away, and not only conveyed southward, but, as Agassiz first pointed out, distributed northward, though only at comparatively slight distances. Long Island, that narrow fork of land running eastward and separated from the southern shore of Connecticut by the Long Island Sound, a shallow and turbulent trough, is lined, alike on its southern and northern edges, with bowlders, while its backbone of low hills is also strewed with their débris. They occur gathered together in groups forming topographical features in the landscape, and single ones have a weight of 2,000 tons. As regards their origin, they seem to have drifted from three localities, from the Helderberg Mountains in Central New York, from Manhattan Island, and from various points in Rhode Island, Connecticut, and Massachusetts. Those about the west end of the island may be traced to the Eastern States lying to the north, while many of the western visitors appear to have approached along the valley of the Hudson from the Highlands of New York. On Staten Island we may trace still farther the current of traveled rocks, and find both centres of emigration represented: the bowlders, now gray with lichens, half emerging from the soil, or deeply buried beneath gravel, clay, and sand, or else dispersed in colonies over the surface like pebbles on a board. Manhattan Island, along its southern shore, has been dotted with bowlders of serpentine, dragged from Hoboken, while gneiss and anthophyllite from the bed-rock of the island, limestone from Kingsbridge, and jasper from the Palisades, have likewise been sown across it, though before the restless advance of population they are fast disappearing. In Westchester, Putnam, and Orange Counties, along the banks of the Hudson, these bowlders, all indicating northern extraction, are repeatedly found, frequently at heights of 1,000 feet. These erratics have come from the Shawangunk Mountains, from Whitehall, Essex County, and from Potsdam; in short, they are witnesses of an invasion of northern material prevalent over the State. That these rocks belong northward is not difficult to prove. The reasoning is simply this: When anthophyllite, for instance, a rock unknown in situ to Long Island, appears there in broken and detached masses, we must conclude it belongs to the nearest deposits of the same rock, where it occurs in place, as upon Manhattan Island, and the horn-blendic rock, the gneiss, trap, and iron-ore, similarly found on Long Island, we refer to those conspicuous and well-known localities in Connecticut and Rhode Island where exactly these rocks, identical in chemical composition, are quarried. In the West the same tale is repeated. Throughout Ohio, bowlders are found which are composed of rock utterly foreign to their present surroundings; indeed, of material not known within the limits of the State. These are found perched over declivities, buried in the soil with their exposed edges showing above the surface, or else lying unencumbered in slight depressions of the ground. In Indiana, Michigan, Illinois, Wisconsin, etc., they are omnipresent, and the streets of Cincinnati are paved with the smaller specimens that crowd in exhaustless trains upon the footsteps of their larger companions. In short, we gather the irrefragable testimony, wherever we look for it, through our Northern States, through Europe and Asia, and even along the western coast of South America, that some immense force has been exerted in times past, not only to dislocate and shatter the rocky barriers which opposed it, but also to carry away the evidences of its ravages, and scatter them in its southward movement far removed from their place of origin. Further, let it be remarked that, though one class of these erratics is composed of angular and unworn stones, another yields bowlders that have undergone severe attrition, and along their larger axes are striated and polished; bearing in mind, moreover, that the direction of their transit coincides with that of the furrows and flutings in the same region, we may strictly conclude that they are a feature also of the same excessive and gigantic system of erosion.
But there is another appearance which we believe vitally connected with these, and one of a yet broader and more significant character in its general relations than they are. Over Scotland, England, Ireland, Scandinavia, Denmark, Central Europe, Switzerland, Russia, France, Spain, and in North and South America, in short, wherever we discover bowlders and grooved surfaces, we find a deep and characteristic deposit, not the work of alluvial formations or recent detritus, for it underlies these, but the record of a vast disintegration which, having planed and corroded the continents, has covered the land with sheets of gravel, clay, silt, and sand, all intermixed with stones and bowlders, variously combined in their order of succession, and ranging in depth to over 300 feet. These immense beds furnish gravel for roads and ballast, sand for glass-making and mortars, and clay for pottery; their included stones and fragments are scored and embroidered with fine and interlacing striæ, and they cover the furrowed surfaces of either hemisphere for miles.
They represent the accumulated wear and tear of continents, under some extraordinary agent of erosion and denudation, whose teeth have resistlessly ground upon the solid rocks of the hills and highlands, hiding disfigured surfaces beneath a covering of ruin. Long Island is itself but one long dirt-heap: an accumulated pile of continental débris, sand, clay, gravel, intermixed and overlaid by bowlders, is here gathered together into a more or less stratified state, as if, in an enormous denudation of New England, the aggregated material, scoured from its hills and valleys, had been dropped just upon their outskirts in this long detrital barrow or mound. Yet over New England this same deposit is wide-spread; it lies up and down the valleys, it forms the terraces of its rivers, the shores of its lakes, and, spread over the face of the land, is frequently the immediate soil beneath the feet. This member of the geological series, exhibiting various phases in its deposition, from the bowlder-clay to the lake-ridges, is widely distributed, indeed is universal over the Northern States, and as far south as 40° north latitude extends its sheets and centres of pebbly and sandy deposit in mounds and ridges, themselves capped with accidental bowlders, and resting upon the furrowed and seamed surfaces of the rock beneath. Sometimes they may be found collected in heaps and walls at the foot of the polished rocks, as if silent and incontrovertible witnesses of their severe and prolonged erosion.
In Scotland it is the till, a stiff clay interspersed with polished stones, crowding down the valleys and prevalent over the lower slopes, varying in its lithological character with the character of the surrounding rocks. Gravel and sand beds are intercalated with it and superimposed upon it. In England, Ireland, Scandinavia, Switzerland, we discover identical strata—strata which, while yielding different subdivisions, in their entire extent are the same thing, and only varied according to the local force and extent of the wearing agent, the local peculiarities of the country over which it operated, and the effect which submergence beneath the sea had in redistributing and rearranging the beds of detritus already laid down. In the sequel we shall more particularly revert to this drift-material, and indicate the part it has played in the economy of our landscape-changes; how it constitutes the terraces of our rivers and the successive beaches of our Great Lakes, and how it has choked up the former courses of rivers, forcing them to find new ones by larger and circuitous deflections. Associated with this phenomenon are the appearances known as crushed ledges and roches moutonnées, both of which testify to the exertion of enormous pressure—the one of pressure continuous and progressive, the other, perhaps, of percussive and intermittent attacks.
Crushed ledges designate those plicated, overthrown, or curved exposures where parallel laminæ of rocks, as talcose schist, usually vertical, are bent and fractured as if by a maul-like force battering on them from above. The strata are oftentimes tumbled over upon a cliff-side like a row of books, and rest upon heaps of fragments broken away by the strain upon the bottom layers, or crushed off from their exposed surfaces. Roches moutonnées are those rounded and swelling prominences, often seen in a landscape, which, when examined more closely, show themselves to be truncated masses of rock whose asperites have been smoothed away by the same agency which has planed the rocks everywhere. Only the roches moutonnées have been left furrowed and scratched upon one side, whence the abrading and engraving tool advanced, but vipon the other unscored, and hidden beneath a tail of fragments ground from their opposite slopes. The significance of this we shall see later.
Thus, imperfectly described, we have reviewed the most prominent features of a comparatively modern period, viz., the widely-grooved and polished condition of northern rocks, especially hard-grained rocks, which retain these impressions; the occurrence of wandering bowlders, transported longer or shorter distances from their primitive sites, and the detrital matter from continental abrasion deeply burying the rocky face of the country, and in ridges, mounds, and sheets, extending east and west, and along the greater water-courses, stretching itself down into the Southern States in irregular tails and projections. We will now venture to examine the theories advanced to explain these singular phenomena, and describe that one which best accounts for these facts, with many correlated ones, offering an hypothesis which rationally secures their complete and harmonious agreement.