Popular Science Monthly/Volume 20/December 1881/North America in the Ice Period
| NORTH AMERICA IN THE ICE PERIOD. |
By C. H. HITCHCOCK,
PROFESSOR OF GEOLOGY IN DARTMOUTH COLLEGE.
FEW geological subjects have been discussed so much as the nature, extent, and cause of the glacial period. At first, the speculations of such men as Dr. Buckland upon the ice-markings excited derision, and led to the publication of caricatures. Next, when its claims could not be set aside, some thought it the same with the flood of Noah, and others believed it to represent the chaos supposed to have pervaded space just before the advent of man. Then it was fashionable to believe in a submergence of the land by an ocean freighted with enormous icebergs, floating southerly from the pole. After this the battle raged fiercely between the advocates of icebergs and glaciers, the odds resulting in favor of the glacialists. Agassiz, with his polar-cap theory, advocated the view of a globe entirely encircled by ice, thickest at the poles, but covering the tropics with sufficient thickness to destroy the Tertiary life. Since these days of controversy, geologists have been accumulating facts in all glaciated countries; extreme views are being modified by their holders; and the time has come for consistent, reasonable generalizations in respect to the origin and extent of glacial phenomena.
Where the glacial markings have been most studiously investigated, it appears that an ice-sheet invariably occupies a definite area; that a mer de glace accumulates in the higher central portions, from which glaciers move outwardly in all directions, extending as far as the descending ground will permit, or the oceanic currents can convey the icebergs broken off from the frozen mass without melting; and that these ice-streams carry with them fragments of rock scoring the ledges as they pass along, thus affording the means for determining the exact dimensions of the glaciated areas, even after the climate has moderated and melted the ice. In Europe, the familiar Alpine district is constantly quoted as an illustration of the nature of a glaciated region, with its center of dispersion. Small ice-tributaries now cover the slopes of the Rhône Valley, where formerly the solid mass pressed down from the Bernese Oberland and the region of Mont Blanc, over Lake Geneva and the broad valley of Switzerland to the flanks of the Jura, forming an ice surface fifty miles wide, one hundred and fifty miles long, and two thousand feet deep. On the south, other streams have conveyed morainic materials equally far into the valley of the river Po. Hence we can assign to this area, with its center of dispersion, a definite number of square miles, linking together the actual glaciers with the traces of their former greater extension. No one will question the correctness of the generalizations enlarging the limits of the Alpine glaciated area.
We proceed next to consider the proper extent of the Scandinavian district, which will furnish an example more nearly analogous to that of Eastern America. Erdmann's map in Geikie's "Ice Age" represents striæ pushing northerly into the Arctic Ocean from Finland; southeast toward the White Sea and the Gulf of Finland; southerly in the south part of Sweden, turning to southwesterly toward the Cattegat; west, southwest, and northwest in Norway. Croll, in his work on "Climate and Time," supplements this map by drawing lines to indicate the direction taken by the ice after leaving Finland and Scandinavia. The more eastern lines penetrate the edges of Russia and Prussia. Those passing into the Gulf of Bothnia and the Baltic Sea curve westerly so as to extend over the North Sea, English Channel, southern England, and the north of Scotland. England and Scotland are made to send off additional currents; and the limit of the ice sheet reaches to west longitude 14°, or where the shallow water begins to deepen. The chart is designed to show the probable path of the ice in Northwestern Europe during the period of maximum glaciation. This area is about seventeen hundred miles wide and fifteen hundred from north to south, and will hence compare favorably in size with that of Eastern America. The phenomena of dispersion are explicable upon the theory of the origin of the current from the central Scandinavian ridge, supplemented by radial streams from the Scotch and Welsh highlands; and there is no evidence, unless upon the outermost Hebrides, of any flow of polar ice into the European district. The ice masses have proceeded from higher to lower levels for the most part, rising somewhat to pass over Denmark and southern England; and there is no essential difference between the Alpine and Scandinavian areas save in size.
All the glaciated districts can now be referred to definite areas, separate from one another, and exhibiting grooves radiating from central points and lines. They are, first, the small Alpine district; second, the Scandinavian, with the British additions; third, the paleocrystic sea of the polar regions; fourth, Greenland; fifth, Eastern America, concerning which more will be said in the sequel; sixth, the Rocky Mountain areas, in which there were several groups of glaciers, confluent north of the Columbia River.
Maps will show how the several glaciated regions stand related to one another. They are isolated, disconnected, though all situated upon one hemisphere. Each was a center by itself. The ice moving from the central highlands made its way radially in all directions, carrying detritus and scouring the ledges. The larger ones do not vary greatly in their territorial dimensions. That of Eastern America exceeds the others, especially if we add to it the Arctic archipelago, where, owing to the high latitude, the mer de glace is nearly universal with no great amount of motion. But we do not discover from this delineation any confirmation of the notion of a general polar ice-cap, which has furnished the ice for both the European and American glacial sheets. The Scandinavian, Greenland, and American regions seem to have been entirely independent of each other—separated by deep ocean-water. All of them must have undergone their intense frigid conditions in post Tertiary times, because of some astronomical cause affecting the northern hemisphere equally throughout. We do not find distinct glaciated areas upon the Asiatic side of the pole, because the relations of land and water are unfavorable to the formation of ice-sheets, but the temperature is and has been low enough.
We know but little respecting the ice-sheet about the pole. It has been fashionable to speak of an open polar sea, but no one has yet penetrated into it. The impression derived from the latest Arctic voyages of the American, English, Austrian, and Swedish expeditions is unfavorable to the existence of permanent open water. Yet there may not be an accumulation of a thick, frozen sheet, as the ice-rafts floating away from this northern waste are floes rather than bergs. Nares concludes that this ice is of great antiquity—that it is a paleocrystic rather than an open sea. We need additional observations to satisfy us that the climate is not more severe at the pole than on either side, and whether the meteorological dogma of two poles of cold is correct.
Greenland.—The study of the Greenland glaciers takes us a step further toward the understanding of the American ice-sheet. We have hardly appreciated the size of this island—it being larger than the Alpine glaciated tract. It is over twelve hundred miles long, and four hundred broad, or as far as from Boston to the Missouri River. The interior is covered by a field of ice never crossed in a direct line by any civilized being. From three points attempts have been made to learn something of its nature. In 1830 Keilsen went eighty miles inland from Holsteinberg—latitude 67° reaching the edge of the ice sheet, which could not be climbed. In 1870 Nordenskiöld went in a distance of thirty miles, reaching the altitude of twenty-two hundred feet. He observed that the ice rose gradually toward the interior. The outer edge is a high, precipitous wall. Once entered upon the broad surface of the ice, it is like traveling upon the sea, away from all sight of land. From north Greenland, Dr. Hayes penetrated to a distance of seventy miles. It was a day's journey from the sea to the Avail of ice. The second day was spent in climbing to the table-land; the third day allowed a progress of thirty miles, the angle of ascent falling from six to two degrees. On the fourth day an altitude of five thousand feet was attained, and the ice still continued to rise, but, because of inclement weather, no further progress was practicable. The view was that of a frozen Sahara, immeasurable to the human eye.
It is probable that Greenland slopes westerly in general, having its main axis of elevation near the eastern border. It may be compared to a broad platter inclined westerly, with occasional chinks in the sides through which the ice discharges itself as if it were a viscous body. The principal discharge of icebergs is upon the western side into Baffin's Bay. Not less than thirteen glaciers are found upon the western side to the south of Upernavik, about 73° north latitude, and the largest ones occur farther north. Some of the bergs are three thousand feet hick. The Humboldt glacier empties into Smith's Sound with a width of sixty miles, and showing ice-cliffs from fifty to three hundred feet high above the water. The rock-cliffs adjoining are from five hundred to one thousand feet high. At Polaris Bay a northward transportation is indicated, where Dr. Bessel found numerous granitic blocks containing peculiar garnets, such as abound in south Greenland, resting upon Silurian limestones. Other glaciers have been mentioned farther north. Some have suggested that a series of islands will be found underneath the ice. Little is known of the eastern side, because it is practically inaccessible.
The general trend of Greenland is northwesterly. Hence the south end is farther east than any part of the western coast-line. The oversight of this simple fact has led to confusion among historians and statesmen. The island was discovered by Gunnibjörn in 872. In 983 Eric the Red, banished from Iceland, established a colony near the south end of Greenland, on its western shore, and gave to the island its name. The settlement prospered, and the indications of civilization left behind by these Norsemen exist as far north as Upernavik, or as far as the stoutest ships of modern times can sail without encountering serious risk. The population increased sufficiently to require the services of a bishop, and a list of seventeen terms of clerical office, from 1126 to 1406, has been preserved. A change of political relations led to the destruction of the commerce between Greenland and Scandinavia. Deserted by their friends, pirates and the Skraellings, or Esquimaux, completely exterminated the inhabitants. A record has been preserved in the Icelandic annals of the murder of the very last family in the fifteenth century. For some reason the Danes misunderstood the history after the removal of the restrictions upon the commerce, and many expeditions are said to have been sent out by the Government in search of their old colony in east Greenland, looking for it upon the coast facing Iceland rather than Baffin's Bay. It was thought that the descendants of the original settlers might still be found there, though for many years shut out from communication with Europe; but every ship returned baffled in its attempt to reach the eastern coast. Modern antiquarians have discovered the buildings erected by the Norsemen, read the inscriptions upon the churches and tombstones, and, by a study of the Icelandic sagas, deciphered the whole history, of which an outline has been already given. The facts have also been brought out prominently by Dr. Hayes in a small book describing some recent summer experiences in Greenland with a party of tourists.
All the Greenland glaciers become confluent in the interior, so that it is not a typical mer de glace, but a sheet of ice, which overspreads the highest mountains. It is only near the edge that the several streams manifest their independent existence. Thence the usual phenomena of Alpine glaciers manifest themselves. The sub-glacial streams pouring into the sea produce certain peculiarities. Immense supplies of heat penetrate the ice from the sun's rays, which melt very much ice and discharge muddy torrents of great size. As all water seeks the lowest levels attainable, these currents will find a place at the bottom of the ice-sheet, and wear away the ground-moraines and other débris. Hence banks of earth or clay will be found continuous from the ice-cliff to as great a distance toward deep water as the currents have power to transport the material. Marine animals live in and upon these banks, and leave their remains in them. Hence the deposit is analogous to that series of marine clays called Champlain by me in 1861, and occurring so plentifully up to three hundred feet in the St. Lawrence Valley, and to one hundred and sixty feet along the coast of northern New England and the Provinces. It is obvious that they were contemporaneous with the glacial moraines upon the land, and not entirely consecutive, as so many have supposed. This was the position assumed by the late Professor Agassiz, and confirmed by our own published observations in the "Geology of New Hampshire," describing the occurrence of fossiliferous beds between the lower and upper till of Portland, Maine.
The Eastern American Ice Area.—The latest generalizations indicate that some part of the Labrador Peninsula may be considered as the center from which the ice has radiated over the Dominion of Canada and the northern United States east of the Rocky Mountains. By regarding the Greenland and polar areas as independent of the Labrador sheet, though possibly confluent at the time of maximum glaciation, a multitude of difficulties are removed, and our American glaciers are seen to have been subjected to the same laws as the several ice-fields of other parts of the world. This area extends from Baffin's Bay to Dakota, and from Hudson's Bay, or the Arctic Archipelago, to a line drawn from the Great Banks of Newfoundland through New Jersey, southern Ohio, etc., to the plains east of the Rocky Mountains.
Most of this territory exhibits a southwesterly course of glaciation. This is well shown over the highlands between Hudson's Bay and the St. Lawrence Valley, the valley itself, western New York, Ohio, Iowa, Minnesota, Manitoba, and so on to the extreme western limits. It is very prominent from Lake Superior, near the international boundary, westward to the Rocky Mountains. In eastern New York and the Champlain and Hudson Valleys the course is southerly. In New England the dominant direction is southeasterly, and the same is true of New Brunswick and Nova Scotia. The exceptional deviations are due to local influences, exerted in the decline of the age. The facts in hand for Newfoundland are too few for a satisfactory delineation, while not in disagreement with these generalizations. On the east coast of Labrador there arc several fiords, as if there had been an ice sheet upon the upper part of the peninsula, moving northeast and east. Professor O. M. Lieber's sketches in the Coast Survey report suggest a local glaciation, and not such a general smoothing as would be manifested in cast' the ice had come from the opposite Greenland coast. Packard describes glacial markings in the Hamilton Inlet fiord running to the northeast, and thinks that the movement was to the southeast on the southern coast, or toward Newfoundland.
Farther north, the Meta incognita just north of Hudson's Straits shows an extensive mer de glace, with southerly-moving glaciers. McClintock describes bowlders at Leopold Harbor (North Somerset) and at Graham Moore Bay (Bathurst Island), which have been transported one hundred and one hundred and ninety miles northeast and northwest. Glaciated conditions are mentioned by various explorers to the west of Hudson's Bay, but we have no facts to indicate the direction of the movement. Several statements made in Franklin's second voyage imply a westerly movement.
There is a marked difference in the distances to which bowlders hsve been transported by the southwest and southeast currents. The average distance of the transportation in New England is from twelve to fifteen miles, and no bowlders, so far as known, have been carried more than one hundred miles. At the extreme north edge of Maine, as well as near the north line of Vermont, west of the Connecticut water-shed, are a few bowlders that have come from beyond the St. Lawrence, thus indicating a southeast movement across the St. Lawrence Valley, at right angles to what is supposed to be the common course. In Ohio many stones have been transported more than one hundred miles. In Iowa and Wisconsin bowlders of native copper occur from three hundred to four hundred and sixty-five miles away from their supposed source in Michigan. From the Lake of the Woods G. M. Dawson has described a transportation toward the Rocky Mountains of seven hundred miles. At Baton Rouge, Louisiana, are fossils, perhaps transported by floating ice, that seem to have come from Canada West, as much as twelve hundred miles; and at Natchez, Mississippi, there has been found auriferous quartz, supposed to have come from Montana, eighteen hundred miles. The southwest direction has therefore afforded the examples of the greatest distance traveled by the bowlders. Perhaps topography has aided the result, and the St. Lawrence valley is directly continuous with the Western prairies and Mississippi bottoms, and the New England mountains have intercepted the material brought from the Laurentian highlands.
The evidence is clear, however, of the passage of the ice-sheet directly over all the higher New England summits in a southeasterly direction. The facts illustrative have been specifically given for the Green Mountains, as Mounts Mansfield, Camel's Hump, Pico, Eolus, etc.; for Grey lock, the highest of the Massachusetts mountains; for Mount Washington, and others of the White Mountains, in New Hampshire; and for Katahdin, in Maine, in the several geological reports of those States. The most important case is that of Mount Washington, both because of its greater altitude and because it has been generally supposed to have been an exception to the rule, and most geologists have spoken of it as an island in the ice age.[1] I have fully stated elsewhere[2] the facts proving the presence of the ice-sheet there. They are, briefly, the usual glaciated appearance of the ledges, striæ, and transported bowlders, two of them weighing nearly one hundred pounds, and distinctly identical with ledges several miles distant. All the surface rocks above five thousand feet have been riven into angular fragments by the long-continued action of frost, insomuch that the embossment and striation have been nearly obliterated, and the fragments so covered by lichens that extraordinary care is required to discover any dissimilarity among them. Washington is the highest peak southeast from the St. Lawrence Valley; hence, if that has been glaciated by ice moving from the northwest, then every part of New England has been covered by the same sheet.
Assuming it well established that the center of dispersion for the Eastern American drift was in the west part of Labrador, we are met by the difficulty that the land beneath this central mer de glace is not so much elevated as the New England mountains which have been covered by the ice originating in those Laurentian highlands. As to the facts, they are indisputable—the glacier has moved over slopes higher than the mountains at its source; and it may be that we have something new to learn from these facts about ice accumulation and movement. Professor Dana has proposed the theory of a change of level in the land since the ice period. Assuming the starting-point to have been in the Laurentian highlands, if a descent of ten feet per mile be allowed, the sheet must have been at least thirteen thousand feet high to have allowed it to slide over Mount Washington; or, the land must have been four thousand five hundred feet higher than now.[3] The distance from these highlands to the base of the Rocky Mountains along which the ice is known to have moved southwesterly is about fifteen hundred miles, requiring, on the basis of Professor Dana's data, an elevation of certainly four miles. A change of level to that extent seems improbable.
Recent writers have developed a molecular theory of glacier motion; and in connection with it mention such possibilities of ice-accumulation as to suggest a method of relief from the difficulty of understanding how the ice can move up an elevation. The mere weight of the ice does not cause it to slide downward. According to Canon Moseley, from thirty to forty times its weight is required to shear the ice; the motion is proportional to the amount of heat present. The melted ice is very susceptible to the action of gravity, and the motion is greatest where the most heat is manifested, or upon the south side of the glacier.
The writings of Moseley and Croll recall the utterances of Agassiz as respects the motion of the glacier, lie himself modestly avows his physical researches upon the glacier as inferior to those of his successors, Forbes, Tyndall, etc.; yet he could not shut his eves to the fact that those physicists had not satisfactorily solved the problem. While ascribing the motion to expansion by freezing, Agassiz insisted that heat was largely concerned, asserting that its presence was more consequential than that there should be an inclined plane—that the ice might move up-hill toward the sun; and particularly that a great thickness of it could make its way over such level territory as the prairies of Indiana and Illinois. The ice along the southern melting edge would be charged with moisture percolating through the pores and capillaries, descending the icy slopes obedient to gravity, and no longer requiring the shearing force. The amount of motion would in these circumstances be like that of the Greenland sheet, sixty feet per diem, rather than the sluggish crawl of one to three feet in the same time of the comparatively poorly developed Alpine glaciers. In agreement with these views we find the motion northward toward the pole to be very slight, though the land may be inclined northerly as in Grinnell Land.
Now, if we apply these principles to the territory in question, we would say that the ice began to accumulate in Canada from an unusual precipitation of moisture, gradually filling up the St. Lawrence Valley, and at first moving southwesterly. But, the supply still continuing plentiful, the valley fills up and runs over. It does not need to accumulate to the thickness of several miles upon the Laurentian highlands so as to have a downward slope all the way to Mount Washington. A mass only a few thousands—possibly hundreds—of feet thick might soften before the southern sun and the influence of the Atlantic Ocean off our coast, and lead the plastic material southeasterly over the Montalban water-shed. Once started upon the seaward slope, the ice could not fail to reach its destination. The southwest motion would likewise continue, and accomplish greater results, transporting blocks much farther because commencing earlier, continuing longer, and pushing forward in a thicker sheet. This Canadian ice would have resembled the present mer de glace of Greenland, confluent over hill, valley, and island, dragging the reluctant erratics up hill and down, accumulating ground moraines and lenticular hills, hollowing out pot-holes and discharging clouds of mud into the edge of the sea. Hence, instead of saying that the land rose three or four miles above its present level in Labrador in order to give the required impetus to the ice-movement in New England, it is easy to see how the same work could be accomplished by the action of much simpler causes.
Messrs. Torell and Dana have advocated the notion that the Greenland and Eastern American areas are one, and that Greenland was the source of the ice that has covered the eastern part of our continent. One can not see that this view better suits the facts than to suppose there were two centers of dispersion, one for Greenland and the other for the continent. From Greenland to the southwest edge of the glaciated area the distance is twenty-five hundred miles, two fifths greater than from the Laurentian highlands, and requiring a descent of over seven miles of vertical elevation from the center to the circumference. It seems unnecessary to add to the difficulties of height incurred in the shorter distance. Provided there has been no great elevation of the land, Davis' Strait seems to contain enough ocean-water to carry off all the glacial products poured into it from either side. The facts of ice movement already stated for Labrador and the Arctic Archipelago show motion easterly and northerly in the teeth of this imagined current from Greenland. Hence, as we find phenomena of glaciation in agreement with our view of radial dispersion from Labrador, whose existence was unknown to Torell, it seems as if it were altogether unnecessary to look so far as Greenland for the source of the ice-flow.
Terminal Moraines.—Years ago, those who believed icebergs would explain glaciation triumphantly asked the glacialists, Where are the terminal moraines which must have accumulated at the lower edge of the great ice-sheet? It is very strange their existence was not suspected by the early glacialists; and, as we now show their lines of distribution upon our maps, we remove another obstacle to the acceptance of the glacial theory.
Writers now generally employ the word till to denominate the materials accumulated by the ice, including the moraines and bowlder clays. The ground-moraine is that form of the till least noticed in the examination of active glaciers, because situated in the nearly impenetrable abysses between ice and earth. In the continental glacier, where the surface had the unbroken white snow for its covering, this form of moraine accumulation must have been the most abundant. In middle New England the ground-moraine is developed into the lenticular hill—an oblong rounded hummock, sometimes two hundred feet high, mostly composed of lower till, with a trend corresponding to the direction of the ice-current in the neighborhood, varying from nearly southeast near Newburyport to south 10° west in the Connecticut Valley. This lower till is compact, sometimes clayey, full of small, scratched, far-traveled stones in a forced position, with the iron coloring matter in the ferrous or protoxide condition. The capping of the hill or upper till is loose, the fragments are rough, not far removed from their source, commonly lying naturally, and the color is yellowish red from the presence of ferric oxide. These characteristics suggest the derivation of the upper till from the materials held in the ice at the time of its melting; they falling promiscuously upon the surface of the ground-moraine, compacted by the great weight of the glacier.
The moraines regarded by us as terminal are in all respects like the upper till, as would be expected, considering that the fragments in front of the glacier principally consist of extensive tracts of that deposit crowded into a small compass. In connection with it one often sees sloping plains of gravel and sand deposited by streams of melted ice acting upon the moraine, or, if the supply of water has been copious, the unstratified drift has all been modified. Some authors think the upper till is only the surface portion of the ground-moraine acted upon and oxidized by atmospheric agencies. If the difference in the angularity, roughness, and distance traveled of the stones is not sufficient to justify our definitions, then these writers must explain why our terminal moraines should show the oxidation throughout their whole mass, hundreds of feet thick, while the upper till is usually very thin, often no deeper than the roots of large trees.
There are two lines of terminal moraines more conformable to each other than to the extreme southern limit of the glacier. East of Cape Cod this line is supposed to have passed over the St. George's and Great Newfoundland Banks, while the icebergs carried débris from the land to unknown distances southerly over the Atlantic Ocean. The outermost series of terminal moraines commences upon the Island of Nantucket, and is traceable thence across Martha's Vineyard, Block, Long, and Staten Islands; whence, according to Professor Cook, "the whole line of the moraine [across New Jersey] is remarkably plain and well defined." The line across Pennsylvania has been traced out recently by Mr. H. C. Lewis. This whole series, as far west as Ohio, occupies the outer margin of the glaciated area; and Professor Cook thinks the same moraines will be found conterminous with the extreme southern limit of the ice-sheet to its remotest bound in Montana.
The inner line of moraines starts at the middle of the east coast of Cape Cod, follows the curved shore to old Plymouth, thence south to the Elizabeth Islands on the border of Buzzard's Bay. After passing under water for several miles, it comes to the surface along the south shore of Rhode Island west of Narragansett Bay, touches Plum and Fisher's Islands, continues to the northeast angle of Long Island, passes through the greater part of Brookhaven, Riverhead, and Southold, where it disappears.
The outer moraine determines the topography of Long Island, as it constitutes a marked ridge, or "backbone," as sometimes called, from Montauk Point to Fort Hamilton. The highest point is three hundred and eighty-four feet, and the base of the moraines is usually more than fifty feet above tide-water. The southern slope is a gently inclined sandy plain, made of the ruins of the terminals; the northern slope terminates in cliffs, because so largely consisting of cretaceous clays. The eastern and middle portions of this ridge consist of modified drift, containing few or rare bowlders. The western portions represent the typical constitution of terminal moraines, and are well shown in the numerous excavations made for building purposes about Brooklyn. The bowlders upon Long Island have all been brought from the northwest, and those familiar with the ledges in Connecticut and up the Hudson can readily recognize the fragments in the various parts of the island—the Palisade traps in Brooklyn, and the New Haven red sandstones to the southeast of their place upon the mainland.
We can recognize the inner moraine in Ohio, while its remnants may be found in New York State by future investigators. Dr. Newberry describes a line of kames occupying the water-shed between the affluents of the Ohio and Lake Erie, which can be easily correlated with the modified drift fringing our terminal moraine. This line extends entirely through Ohio, and bends sharply at Fort Wayne, Indiana, following St. Joseph's River northeasterly, so that its whole course is parallel to the shore of Lake Erie. N. II. Winched thinks there are several of these moraines in the northwest part of Ohio. Professor T. C. Chamberlin has generalized the facts about the course of these moraines between Pennsylvania and Minnesota, and supposes there are two morainic lines parallel to the shore of Lake Erie, the outer reaching to middle Indiana. Likewise there appear moraines in the form of loops following the course of the shores of Lake Michigan, Green Bay, Keweenaw Bay, and the southwest end of Lake Superior. The glaciers seem to have followed the several valleys, continuing to flow as long as the material lasted. The Green Bay and Lake Superior streams did not cover the area in the lee of the highlands of northern Wisconsin; and hence there was a large tract of land, occupying essentially what is known as the "Lead-region of the Northwest," over which we search in vain for erratics or glaciated surfaces. Mr. Warren Upham has communicated to us many facts for Minnesota, Dakota, and Iowa. They indicate two looped moraines west of the Mississippi: one reaching nearly to Yankton, Dakota, having the celebrated Coteau de Missouri for its western border, and part of the Coteau de Prairie for its eastern; the other taking the eastern line of the Coteau de Prairie for one side, and pointing east of south to Des Moines, Iowa. An inner loop connecting the Wisconsin moraine with that of the Leaf Hills may have been pushed there by the Lake Superior stream, and the more southern loops may have had some connection with glaciers starting in the Dominion portions of the Rocky Mountains. Hence the Labrador and Rocky Mountain sheets may have been confluent, and, owing to the great masses of ice thus accumulated along the upper part of the tributaries of the Mississippi basin, we may understand why the glacier extended so much farther south in the interior than upon either coast.
Elevation of the Land in the Glacial Age.—The earlier writers accounted for the glacial cold by supposing the land had been elevated sufficiently to lower the temperature, and subsequently depressed to its original level. Later authors suggest that the formation of ice-caps at the poles would draw away enough water from the lower latitudes to increase the amount of land in the north. Either view would afford conditions adequate to produce the results; but the evidences of such an oscillation, from the nature of the case, are very difficult to obtain. The most obvious are derived from the existence of ancient river-channels now submerged beneath the ocean. Upon the south sides of Long Island[4] and Cape Cod[5] there are several ravines channeled out of the till and modified drift, too large to have been excavated by the present drainage system. They are fiords, and many of them are filled by linear fresh-water lakes, kept in position by bars of beach-sand near the ocean's level. Similar facts have been reported in connection with the former entrances of tributary streams into the Great Lakes.[6] In all these cases the excavations must have been made in connection with the disappearance of the ice-sheet.
A more extensive series of excavations occurred in the larger rivers, though it is not so easy to fix their date. By studying the submarine contours off the mouths of the Connecticut and Hudson Rivers, we can follow the outlines of their valleys for many miles out to sea. The first named flowed around the east end of Long Island, leaving the Housatonic to join the Hudson just below Manhattan Island, as shown by Professor Newberry. The Hudson continued southerly for seventy or eighty miles, as first pointed out by Professor Dana. As no deltas appear in connection with the present mouths of these streams, their submergence must have been comparatively recent, while they may have existed as channels of erosion for millions of years.
A further examination of coast-charts reveals the fact that there is a belt of shallow land bordering the continent from New Jersey to Newfoundland, and that it is as wide as the extensions of the Hudson and Connecticut. Hence, if there has been a submergence off New York, the same oscillation occurred along the whole coast; and thus a tract of land, as large as Pennsylvania, New York, and the maritime provinces combined, has been lost to the continent, probably since the glacial period. The corresponding area in Northern Europe, which seems to have been elevated at the same time, may be found indicated in Geikie's "Ice Age."
More impressive proofs of a former elevation of the continent appear from a careful study of the lower Mississippi. The contour-line of one thousand feet depth suggests the continuance of the river-bed for a distance of forty miles into the Gulf of Mexico. The Tertiary rocks of the river-basin have been excavated to the depth of two hundred to five hundred feet, and then unconformably overlaid by the orange sand-a deposit coeval with the glacial drift. Borings in lower Louisiana show first the Port Hudson group or post-glacial deposits; second, the orange sand; and, third, the underlying eroded Tertiaries with their characteristic fossils. This excavation could not have been made unless the waters of the Gulf had been at least five hundred feet lower than at present.
Other facts further illustrate these former elevations. Upon the Atlantic border between New Orleans and New York we find a low, broad plain, largely consisting of marshy or drowned lands. On reaching Virginia this land begins to disappear, but the plain holds the same relation to the continent, as it continues to exist in the submarine banks east of Massachusetts. Curiously enough, we now dredge from the George's and Great Banks Tertiary fossils similar to those occurring in the unsubmerged parts of this plain. Their existence beneath the ocean had never been suspected till the possible identity of the Atlantic plain with the eastern shoals had been suggested, subject to the crucial test of dredging.
These facts authorize us to believe that the eastern half of the continent has been elevated certainly six hundred feet in the glacial period. As this elevation included Northern Europe, and consequently the polar districts, so as to unite the Old and the New Worlds, we may be able to follow the older writers, and find in this land-mass the conditions adequate to produce the glacial cold. Such a cause will explain the facts more satisfactorily than the invocation of the eccentricity of the earth's orbit, or the precession of the equinoxes. If we can combine the two classes of causes, we can certainly explain all the phenomena, besides obtaining the data for the chronology of the ice age.
The Melting of the Glacial Sheet.—Time fails us to describe the marvelous facts connected with the melting of the ice. The numerous kames, elevated sand-plains, and river-terraces, found every-where in Northern America, came into existence with the copious floods of water resulting from the dissolution of the ice. The history of the ice age is incomplete without a discussion of the events occurring in this great continental freshet; but this sketch must be deferred for the present.
- ↑ E. Hitchcock, "Proceedings of the American Association of Geologists," 1841, p. 182. L. Agassiz, "Geological Sketches," Second Series, p. 98.
- ↑ "Geology of New Hampshire," vol. iii, p. 203.
- ↑ "American Journal of Science," III, vol. ii, p. 327.
- ↑ "American Journal of Science," III, vol. xiii, p. 142 (Lewis).
- ↑ "American Naturalist," vol. xiii, p. 555 (Upham).
- ↑ The discovery of the pre-glacial outlet of Lake Erie by Professor J. W. Spencer, and the former northerly drainage of the tributaries of the Alleghany River by the Pennsylvania geologists, have been published since this article was written.