Popular Science Monthly/Volume 30/November 1886/North America in the Ice Period
|NORTH AMERICA IN THE ICE PERIOD.|
PROFESSOR OF GEOLOGY AND PALEONTOLOGY IN COLUMBIA COLLEGE.
ALTHOUGH the glaciated area on our continent has been as yet but partially explored, abundant proof has been gained, as it seems to me, of the truth of the following propositions, viz.:
1. That glaciers once covered most of the elevated portions of the mountain-belts in the West as far south as the thirty-sixth parallel, and all the eastern half of the continent to the fortieth parallel of latitude.
2. That the ancient glaciers which occupied the area described were not produced by local causes, but were the exponents of a general climatic condition.
3. That they could not have been the effect of a warm climate and an abundant precipitation of moisture, but were the results of a general depression of temperature, and therefore afford proof of the truth of what is called the glacial theory.
The facts and arguments which sustain these propositions may be briefly summarized as follows:
The glaciation of the Sierra Nevada is general and very striking. It has been studied by Whitney, King, Brewer, Le Conte, and others, including the writer, who have given abundant proof that all the highest portions of the range were once covered with snow-fields, and that glaciers flowed from these down the valleys on either side. Mount Shasta once bore many great glaciers, of which miniature representatives still remain; the Cascade Mountains exhibit, perhaps, the most impressive record of ice-action known; all the higher portions of the range are planed and furrowed by glaciers which descended into the valley of the Des Chutes on the east, and the Willamette on the west, as shown by my observations in 1855, at least twenty-five hundred feet below the present snow-line. Mount Rainier still carries a dozen glaciers of considerable size, and all the country is glaciated about this and the other snow-peaks of Washington Territory, Mount St. Helen's, Mount Baker, Mount Adams, Mount Olympus, etc., around Puget's Sound, and on Vancouver's Island. In British Columbia, as shown by George M. Dawson, Dr. Hector, James Richardson, and others, the signs of ancient glaciation are conspicuous in all the high country explored. Along the coast farther north the ancient glaciers have left their marks in all the fiords, and those of the present day descend lower and lower until in Alaska they reach the sea-level.
The valleys of the Wahsatch range were once filled with masses of ice as far south as Central Utah. A type of these, though not the largest, was Little Cottonwood glacier, of which the record has been carefully studied by the writer. It formed in a cirque at Alta ten to eleven thousand feet above the sea, where its bed is everywhere conspicuously glaciated. It had a length of about ten miles; its thickness, as shown by the line of granitic blocks, left along its sides, was five hundred feet or more, and its lower end protruded into the Salt Lake Valley at a level not greater than fifty-five hundred feet above the ocean. The glaciation of the Uintah Mountains has been graphically described by King, who says that all the higher portions of the range were once covered with a continuous sheet of snow and ice, and that glaciers descended through all the important valleys; also that the ancient glaciers of the Uintahs occupied a greater area than all those now existing in the Alps.
In the Rocky Mountain belt the signs of ancient glaciers abound from the northern part of New Mexico through Colorado and along the great divide in Wyoming, Montana, and Idaho. In the valley of the Arkansas, particularly about Leadville, in the Parks, on Clear Creek, in the valley of the Rio Grande, roches moutonnées, lateral and terminal moraines, embankment-lakes, etc., all the work of glaciers, have been observed by thousands of travelers. Here the mountain-belt is very broad and high, is now the great condenser from which radiate all the most important streams of the West, and in winter is covered with a heavy sheet of snow. In ancient times it played a similar rôle, only that the snows of winter did not melt as now in summer, but accumulated from year to year until they produced great glaciers. In Wyoming the mountains are narrower and lower, and the glacial signs are less conspicuous; but toward the British line, where the ranges multiply and the summits are higher, the records of glaciation are everywhere apparent.
To summarize the description of the glacial phenomena of this Western region, it may be said that, over all the mountain-ranges north of the limit before given, the traces of ancient glaciation are alike in character and apparently of the same date, and are evidently the effects of general and not local causes. In the country east of the Mississippi the evidence of ancient glaciation is even more wide-spread and impressive than in the far West. The surface-rocks of Canada, New England, New York, and the greater part of Ohio, Indiana, Illinois, and Wisconsin, bear marks of ice-action, and are generally covered with a sheet of drift material which has been carried from the north southward, often many hundred miles. This
glaciated and drift-covered area extends from Maine and Massachusetts in a belt parallel with the arch of the Canadian highlands five hundred miles wide and more than two thousand miles long. Its extension northward from the head-waters of the Mississippi has not been traced farther than Lake Winnipeg, where it was studied by Hind; but there are good reasons for believing that it reaches northward to the Arctic Ocean, and that the great lakes of the north, like those of the St. Lawrence chain, Superior, Huron, Michigan, etc., are pre-glacial river-valleys scooped out and modified by ice.
From the facts already gathered, it is a justifiable inference that fully half the Continent of North America and nearly all north of the fortieth parallel was at one time covered with ice or perpetual snow, and, so far as we can now judge, the glaciation of all the North American localities enumerated was synchronous.
Some writers have attempted to prove that a large part of the glacial phenomena described above is really the work of icebergs and shore-ice, and one of the consequences of a great continental subsidence; but no man who has studied the inscriptions made by glaciers will hold to this theory when he has traversed much of the glaciated areas east or west of the Mississippi. To all the mountainous region of the West it is evident that the iceberg theory is inapplicable, and, when the enormous glaciers of the West are conceded, it is difficult to see why they should be denied to the East. Since, however, the iceberg theory is insisted upon for this section, it may be well enough to say that it is demonstrated untrue by four unanswerable arguments:
1. The inequalities of level in the fancied water-line formed by the margin of the drift area are irreconcilable. Mount Washington, Mount Marcy, Mount Mansfield, must have been totally submerged—because their tops are worn and striated—while the shore-line was at New York at the sea-level, in Pennsylvania twenty-one hundred feet, and at Cincinnati three hundred feet higher. South of the drift-line, high lands and low were alike beyond the reach of the flood, while in Wisconsin it spared a special district not above the general level, and all around it the rocks are scored and strewed with débris.
2. The direction of the ice-scratches and the derivation of the bowlders would require the submergence of all the northern portion of the continent, so that icebergs (which had no land to start from, and therefore could not have existed) could float southward over all the Canadian highlands; and the local variations of direction (southwest by south, in the basin of Lake Erie, south in that of Lake Huron, south-southwest in Lake Michigan, southwest in Lake Superior, and southeast in New England) show an incomprehensible tangle of ocean-currents.
3. The complete absence of marine shells from the great drift area of the interior, while they are abundant in the Champlain and bowlder clays on the coast, is incompatible with this theory.
4. The inscription left by the eroding agent is altogether sui generis, and characteristic of glacial action, and not at all that which could be effected by dragging masses of ice over the sea-bottom. This in itself is a conclusive refutation of the theory. The record made by a glacier is unmistakable, and no one who has not learned the language in which it is written is warranted in taking part in the discussion; but he who has done so will find graven on the rocks of the Alps, the hills of New England, the basins of the Great Lakes, and the mountains of Colorado and Oregon, an inscription which is everywhere the same, which can have but one meaning, and bears a signature that can not be counterfeited.
While it is hopeless to expect that all men will agree upon this—or any other—subject, I think I am justified in saying that the facts which have been stated, and others of like import, constitute an indisputable record, not necessarily of the former existence of a great icecap over all the northern regions, but of the simultaneous prevalence of sheets of land-ice, i. e., glaciers, over great areas of our continent; and that these glaciers, forever in motion, holding imbedded in their substance sand, gravel, and bowlders, pressed against the underlying rock by their enormous weight (probably averaging fifty thousand pounds to the square foot), became powerful agents of erosion; general and uniform when they were broad, narrow and special when they were local. This is the reading of the facts now given by those who are best qualified to judge of the import of the phenomena in the Old World and in the New. Already the belief in an ice period and ancient glaciers is general—hereafter, with more complete knowledge of the subject, it must become universal.
Accepting the facts cited above as demonstrating the truth of the glacial hypothesis, and as proving beyond cavil the reality of an ice period, we now pass to consider the proximate and remote causes of the distinctive phenomena of this remarkable chapter in geological history.
With characteristic conservatism Lyell endeavored to account for the prevalence of glaciers over the northern hemisphere by supposing them to be due to a peculiar arrangement of land and sea; broad and elevated areas of land in the Arctic regions, low and narrow land surfaces in the tropics. I have elsewhere discussed this question at some length, and have shown that this theory is untenable, because: First, during the Tertiary age the land was high at the north, no marine Tertiary deposits being found there; Asia, Europe, and America were then connected by land, and the tropical currents were excluded from the Arctic Ocean, but in that age a warm climate prevailed over all the Arctic regions. At the same time the tropical lands were locally if not generally lower than now, since in the West Indies and on the borders of the Gulf of Mexico are broad sheets of marine Tertiary; Second, because all evidence is wanting of high northern lands during the ice period; and, at least during a portion of the time when an arctic temperature prevailed from New York northward, the sea stood much higher than now, receiving and precipitating the Champlain clays—the fine flour ground by the land glaciers—and burying in them arctic shells.*
In the article referred to I have shown that no terrestrial causes yet suggested are adequate to produce an ice period, and that we are compelled to look to some cosmical cause for an explanation of its occurrence.
Recently a voluminous and elaborate review of the subject has been published by Professor J. D. Whitney, with the title of "Later Climatic Changes," the object of which is to prove that there has never been an ice period, properly speaking. To establish this, it is claimed that ice has little or no eroding power; and the few ancient glaciers, of which the evidence can not be ignored or sophisticated, are considered as the products of local causes. Following Lecoq and others, Professor Whitney claims that since snow and ice are forms of moisture evaporated elsewhere by heat, the extension of glaciers at any time or place is simply an effect of increased evaporation—of heat and not cold—and hence if there ever was an ice period, meaning a time when glaciers were more widespread than now, it must have been a warmer period than the present; forgetting, apparently, that increased congelation is the only necessary feature in the increase of glaciers, for, without this, increased evaporation and precipitation would be inoperative. Only a few of many facts need be cited to show that this theory is untenable: 1. Glaciers are now confined to altitudes and latitudes where the temperature is low—Alpine summits and the Arctic and Antarctic Continents. To extend the reach of the glaciers now existing, and to reproduce them where they existed formerly but are now absent, it would be only necessary to widen and intensify the conditions upon which their existence depends, viz., lower the temperature and cause the present precipitation to be more generally fixed in ice and snow. A single example will be sufficient to prove the truth of this statement. On the Cascade Mountains in Oregon we find a copious precipitation of rain and snow, but no ice where great glaciers formerly existed. The snow-fall is so heavy that the snow-line is brought down to an altitude of seven thousand feet above the ocean, and there the temperature is high enough to permit the vigorous growth of trees and smaller plants. The fir-forests here meet the snow-banks in actual mechanical conflict, and the front ranks of trees, though of good size, are weighed down by the snow and grow prone and interlaced upon the ground. The snow-fields rise three thousand to four thousand feet above the snow-line, and there are miniature glaciers at the heads of the valleys—representatives of the great glaciers that once filled these valleys to their mouths. The precipitation remains, the snow-fall remains, but the glaciers are gone. Here we have just the conditions most favorable to the formation of glaciers according to the theory of those who regard glaciers as thermal phenomena, but no glaciers—because of the high annual temperature. With an increase of the average annual temperature, even with increased evaporation and precipitation, it is evident that no glaciers would form; but with a depression of temperature which should cause the rain-bearing winds from the Pacific to do all the year what they now do only in winter; viz., heap up snow on the highlands; and some of this snow-fall should accumulate year after year, the mountain-slopes and draining valleys would soon be occupied by glaciers, as they were in former times. So if winter conditions could be made permanent on the great water-shed of the Canadian highlands, and the water of the St. Lawrence, the Mississippi and Red Rivers were retained in the form of snow and ice, glaciers would fill again the lake-basins, override the highest summits, and cover with an ice-sheet all the old glaciated areas.
Even if the evaporation from adjacent seas were somewhat diminished by the cold, that would not change the result, though it would prolong the time. The evaporation from the ice-cold oceans in the regions surrounding the north and south poles is now sufficient to produce continental glaciers in Greenland and on the Antarctic Continent, and it requires no argument to show that like conditions would produce like results in what is now the temperate zone.
On the theory that increased evaporation and precipitation would cause an extension of glaciers, and as an illustration of their local origin, it has been suggested that the water with which the dry regions of our Western Territories were once abundantly supplied produced the glaciers of which we find traces on the adjacent mountains, and the exhaustion of the water caused the disappearance of the glaciers. We shall see, however, that this is a speculation which is as yet sustained by no proof. It is well known to all geologists that the interior of the North American Continent has been occupied by a succession of great fresh-water lakes, extending in time from the early Eocene to and through the Quaternary. The history of these lakes has been admirably worked out by King, Gilbert, and Russell. Those of the Tertiary were numerous and broad, providing ample evaporating surfaces, but so far as we know they contributed nothing to the formation of glaciers—which could not have existed, indeed, under the warm sun of the Tertiary ages, except on mountains higher than any the continent now bears. In the Tertiary the climate was sub-tropical over all the area of the United States south of the British line, as is shown by the fact that palms and cinnamon-trees grew as far north as Vancouver's Island and the falls of the Missouri.
The relations which the great Quaternary lakes, Bonneville, La Hontan, etc., bore to the former glaciation of the adjacent mountains is an interesting subject of inquiry. As I have mentioned, it has been suggested that it is the relation of cause and effect, but this is supported by no proof, and opposed by strong circumstantial evidence. The lakes and the glaciers may have been synchronous, and, to some extent, co-operative phenomena; but the relationship was rather fraternal than filial, as they had probably a common parentage.
The cause of the former wide spread of water-surfaces in the undrained portions of the Great Basin was either more copious precipitation or less rapid evaporation than at present. It is well known that the supply of moisture of this region is derived from the rain-bearing winds which blow steadily on to the land from the Pacific, and "the testimony of the rocks" is conclusive to the effect that there has been no change in the outline or elevation of the land, or the relations of land to sea since the Tertiary age, which could have materially increased or diminished the precipitation.
So in regard to the topography of the interior. Since the end of the Tertiary it has remained essentially the same. The hydrographical basins have been filled and emptied, but the old beach-lines which mark their sides prove that the country has remained substantially undisturbed. It is apparent, therefore, that the causes of any variation in the amount of precipitated or accumulated moisture must be climatic and not topographical. King, Gilbert, and Russell have shown that there have been several alternations of wet and dry climate in the Great Basin, and they are substantially in agreement that there have been two wet and two dry periods, of which the last is the present.
It would seem easy to determine by observation the relationship between the lakes and glaciers of that region, since some of the glaciers descended far below the highest water-level, as was the case with the Little Cottonwood glacier, to which reference has already been made, but the actual contact of the glaciated surface and the lake sediments is there covered and concealed by modern débris. The observations made elsewhere by Gilbert and Russell will, when published, probably demonstrate that which can now only be conjectured. "We can confidently predict, however, that it will be found that the same climatic condition which produced the accumulation of water in the lake-basins also caused the accumulation of congealed water on the highlands. A greatly increased rainfall might produce lakes without forming glaciers, but we appeal in vain to the facts or the imagination for a probable cause of an increased oceanic evaporation, with a more abundant precipitation on the land. Hence we seem driven to the acceptance of the other alternative—diminished evaporation—for the filling of the reservoirs of the Great Basin. And when we search for a cause of diminished evaporation only one presents itself, but that offers an easy and natural solution of the problem, A depression of temperature would certainly reduce evaporation (since the power of air to absorb moisture varies directly with the temperature), and at the same time form lakes in the valleys, and glaciers on the mountains. To prove this, we have only to cite the phenomena presented by summer and winter in the Western Territories. In winter the snow-fall on the highlands is heavy, and the accumulation of moisture in this form is large; the skies are cloudy, and the evaporation is small. In summer the sky is cloudless, the heat intense, evaporation and desiccation rapid. In the spring the snows melt, flood the valleys and form temporary lakes, which in midsummer dry up to playas. A climatic change which would perpetuate the conditions of winter and spring would inevitably produce glaciers and lakes, and these would be in the main synchronous; and thus all we find recorded in the past history of this region would be repeated. But to intensify and prolong the summer would not produce either lakes or glaciers.
From the facts which have been enumerated above, it will be seen that from all sides we get evidence confirmatory of the theory that a certain period in the history of this continent was marked by the spread of ice and snow over a very much larger portion of the surface than they now occupy; and that we are fully justified in designating this time as an ice or glacial period; also that this was a period during which, from some extraneous cause, the climate was made colder, and the conditions which now prevail on Alpine summits perpetually, and in winter elsewhere temporarily, were more wide-spread and continuous.
That the Ice period was cold and not warm is also proved by the presence of the remains of an arctic flora and fauna in all regions near the old glaciers; the arctic shells of the Champlain, the arctic plants in the Quaternary clays, the reindeer, the musk-ox, the woolly elephant, and woolly rhinoceros, all tell the same story.
On the preceding pages the Ice period is spoken of as a single geological epoch of the Quaternary age: and so it must be reckoned in any general division of geological time. But the evidence is conclusive that the Ice period was double; that is, there were two maxima of cold separated by a long interval in which the climate was ameliorated, and over large areas which had been for ages occupied by glaciers and snow-fields, the ice and snow were withdrawn, and the surface was covered with vegetation, again to be partially taken possession of by glaciers.
Just how far north the glaciers retreated during the interglacial warm period we do not yet know, but probably not far beyond the Great Lakes; since the vegetation which covered Southern Ohio, during the interval represented by peat-beds between the first and second bowlder clays, was that of a cool climate, and the interglacial beds have not been traced beyond Scarboro Heights, on Lake Ontario.
Facts similar to those from which we have sketched the history of the Ice period in North America, observed in Europe and Asia, afford abundant evidence that the conditions which existed here prevailed over all the northern hemisphere. In South America also similar phenomena have been observed and reported by many geologists. Hence, any explanation offered of the records of the glacial period found here must be comprehensive enough to include the whole great field; and the difficulties which here oppose the acceptance of a theory that is only local in its scope, grow until they become insurmountable. That the conditions which prevailed simultaneously in different parts of the northern hemisphere during the Ice age were synchronous with similar conditions in the southern hemisphere is not proved, nor is it probable that it is susceptible of proof. By many, perhaps most geologists these conditions are supposed to have alternated at the north and south. This much, however, we are justified in asserting, that at an epoch holding the same relative position in geological history north and south of the equator, either simultaneously or alternately, cold climatic conditions prevailed in both hemispheres and left records that are alike in character and import.
An inquiry into the nature of the cosmical influence which we must credit with the phenomena of the Ice period would lead beyond the scope of this paper, and open questions too broad and suggestive to be settled or even adequately discussed in the space at our command. I shall, however, have accomplished the end I had proposed to myself if I have shown—1. That the Ice period was a cold period. 2. That the record of the Ice period on our continent is more complete and impressive than it has been represented to be. 3. That it is the product of general and not of local causes. 4. That these causes were not topographical or even telluric, but extraneous and cosmical.
The question here passes rather into the hands of the astronomer and physicist. The work of the geologist is done when he has shown that the complete solution of the problem does not lie within his domain; that no telluric agency is adequate to produce the phenomena; and that some cosmical cause, such as a variation in the heat radiated by the sun, as suggested by Newcomb, changes in the eccentricity of the earth's orbit, as advocated by Croll, or some other general and all-powerful influence, must be credited with effects as wide-spread and stupendous as those the Ice period has left behind it.
- See position of northern lakes on map.
- Fifty-four thousand eight hundred and ten pounds for one thousand feet in thickness; in some cases (around Mount Washington), probably two hundred and fifty thousand pounds.
- "Popular Science Monthly," July, 1876.
- At least through the channels of the North Pacific or North Atlantic. It has been suggested that in the Tertiary ages a communication existed between the Mediterranean and the Arctic Ocean by way of the Caspian Sea, Sea of Azov, etc.; but if there was an open channel across Western Asia at that time—which has not been proved—it could hardly have been broad and deep enough to permit a flow through it both ways (for no other channel is known) of sufficient volume to modify the climate of the Arctic regions.
- The Champlain clays about New York are near the present sea-level. At Croton Point they are 100 feet higher; at Albany, 200 feet; on Lake Champlain, 850 feet; at Montreal, 500 feet; on Labrador, 800 feet; on Davis Strait, 1,000 feet; and at Polaris Bay, 1,600 feet above the ocean.
- Although this paper is limited in its scope to a consideration of the glacial phenomena of North America in the Quaternary age, and to certain erroneous notions which are entertained in regard to it, it may not be out of place to say that it is believed by many geologists that there have been several ice periods, and one at least as far back as the Permian epoch.