Chapter XIII: The Great Ice Age (2)

When extreme glacialists point to Greenland and ask us to believe that in the Glacial age the whole continent of North America, as far south as the latitude of 40°, was covered with a continuous glacier, having a wide front, and thousands of feet thick, we may well ask, first, what evidence there is that Greenland or even the Antarctic continent is at present in such a condition; and, secondly, whether there exists a possibility that the interior of a great continent could ever receive so large an amount of precipitation as that required. So far as present knowledge exists, it is certain that the meteorologist and the physicist must answer both questions in the negative. In short, perpetual snow and glaciers must be local, and cannot be continental, because of the vast amount of evaporation and condensation required. These can only be possible where comparatively Warm seas supply moisture to cold and elevated land, and this supply cannot, in the nature of things, penetrate far inland. The actual condition of interior Asia and interior America in the higher northern latitudes affords positive proof of this. In a state of partial submergence of our northern continents, we can readily imagine glaciation by the combined action of local glaciers and great ice floes; but in whatever way the phenomena of the boulder clay and of the so-called "terminal moraines" are to be accounted for, the theory of a continuous continental glacier must be given up.

The great interior plain of western Canada, between the Laurentian axis on the east and the Rocky Mountains on the west, is seven hundred miles in breadth, and is covered with glacial drift, presenting one of the greatest examples of this deposit in the world. Proceeding eastward from the base of the Rocky Mountains, the surface, at first more than 4,000 feet above the sea level, descends by successive steps to 2,500 feet, and is based on Cretaceous and Laramie rocks, covered with boulder clay and sand, in some places from one hundred to two hundred feet in depth, and filling up pre-existing hollows, though itself sometimes piled into ridges. Near the Rocky Mountains the bottom of the drift consists of gravel not glaciated. This extends to about one hundred miles east of the mountains, and must have been swept by water out of their valleys. The boulder clay resting on this deposit is largely made up of local _débris_, in so far as its paste is concerned. It contains many glaciated boulders and stones from the Laurentian region to the east, and also smaller pebbles from the Rocky Mountains, so that at the time of its formation there must have been driftage of large stones for seven hundred miles or more from the east, and of smaller stones from a less distance on the west. The former kind of material extends to the base of the mountains, and to a height of more than 4,000 feet. One boulder is mentioned as being 42 × 40 × 20 feet in dimensions. The highest Laurentian boulders seen were at an elevation of 4,660 feet on the base of the Rocky Mountains. The boulder clay, when thick, can be seen to be rudely stratified, and at one place includes beds of laminated clay with compressed peat, similar to the forest beds described by Worthen and Andrews in Illinois, and the so-called interglacial beds described by Hinde on Lake Ontario. The leaf beds on the Ottawa river, and the drift trunks found in the boulder clay of Manitoba, belong to the same category, and indicate in the midst of the Glacial period many forest oases far to the north, having a temperate rather than an arctic flora. In the valleys of the Rocky Mountains opening on these plains there are evidences of large local glaciers now extinct, and similar evidences exist on the Laurentian highlands on the east. A recent paper of Dr. G. M. Dawson on the Palæography of the Rocky Mountains illustrates in a most convincing manner the changes which have occurred in the Cordillera of North America, and the differential elevation and depression which have affected its climate in the later geological periods.[169]

[169] _Transactions Royal Society of Canada_, 1890.

Perhaps the most remarkable feature of the western drift region is that immense series of ridges of drift piled against an escarpment of Laramie and Cretaceous rocks, at an elevation of about 2,500 feet, and known as the "Missouri Coteau." It is in some places 30 miles broad and 180 feet in height above the plain at its foot, and extends north and south for a great distance: being, in fact, the northern extension of those great ridges of drift which have been traced south of the great lakes, and through Pennsylvania and New Jersey, and which figure on the geological maps as the edge of the continental glacier--an explanation obviously inapplicable in those western regions where they attain their greatest development. It is plain that in the north it marks the western limit of the deep-water of a glacial sea, which at some periods extended much farther west, perhaps with a greater proportionate depression in going westward, and on which heavy ice from the Laurentian districts on the east was wafted south-westward by the arctic currents, while lighter ice from the Rocky Mountains was being borne eastward from these mountains by the prevailing westerly winds. We thus have in the west, on a very wide scale, the same phenomena of varying submergence, cold currents, great ice floes and local glaciers producing icebergs, to which I have attributed the boulder clay and upper boulder drift of eastern Canada. In short, we arrive at the conclusion that there never has been a continental glacier, properly so called, but that in the extreme Glacial period there have been great centres of snow and glacial action, in the Cordillera of the west, in the Laurentian plateau of the north, and in the northern Appalachians, and the Adirondacks, while the lower lands have been either submerged, or enjoying a climate habitable by hardy animals and plants.

The till or boulder clay has been called a "ground moraine," but there are really no Alpine moraines at all corresponding to it. On the other hand, it is more or less stratified, often rests on soft materials which glaciers would have swept away, sometimes contains marine shells, or passes into marine clays in its horizontal extension, and invariably in its embedded boulders and its paste, shows an unoxidized condition, which could not have existed if it had been a subaërial deposit. When the Canadian till is excavated and exposed to the air, it assumes a brown colour, owing to oxidation of its iron, and many of its stones and boulders break up and disintegrate under the action of air and frost. These are unequivocal signs of a subaqueous deposit. Here and there we find associated with it, and especially near the bottom and at the top, indications of powerful water action, as if of land torrents acting at particular elevations of the land, or heavy surf and ice action on coasts, and the attempts to explain these by glacial streams have been far from successful. A singular objection sometimes raised against the subaqueous origin of the till is its general want of marine remains; but this is by no means universal, and it is well known that coarse conglomerates of all ages are generally destitute of fossils, except in their pebbles, and it is further to be observed that the conditions of an ice-laden sea are not those most favourable for the extension of marine life, and that the period of time covered by the glacial age must have been short, compared with that represented by some of the older formations.

It follows from all this that the great "continental moraine," which the United States Geological Survey has now "delineated for several thousand miles extending from the Atlantic to the Pacific," cannot be a glacier moraine, but must be, like its great continuation northward, the Missouri coteau, a margin of sea drift, and that we must explain the whole of the drift of the American continent by the supposition, first, of a period of elevation of the hills and subsidence of the valleys in which there were great accumulations of snow on the Western Cordillera; the Laurentian axis, and the Appalachians and Adirondacks radiating in every direction from these points, while minor areas of radiation may have temporarily existed on smaller elevations: that this was followed by a period of more equal level, in which parts of the low grounds were clothed with a temperate flora, the "Interglacial period" so called, succeeded by a second great depression, in which the high level boulders of the second boulder drift were wafted to great distances by floating ice.

The late Prof. Alexander Winchell, a man who did not hesitate to express his convictions, thus bears similar testimony:--"There has been no continental glacier. There has been no uniform southerly movement of glacier masses. There has been no persistent declivity as a _sine qua non_, down which glacier movements have taken place. The continuity of the supposed continental glacier was interrupted in the regions of the dry and treeless plains of the west; and in the interior and Pacific belts of the continent within the United States, ancient glaciation was restricted to the elevated slopes."[170] He might have added that the St. Lawrence valley was submerged and received the ends of Appalachian and Adirondack glaciers on the south-east, and those of Laurentide glaciers on the north-west.

[170] Nov., 1890.

My friend Prof. Claypole, who, however, has some hesitation, fearing, I presume, to be cast out of the synagogue for heresy, ventures to say,[171] "We deduce from the facts and arguments stated above, that all the observations of glacial action in the northern hemisphere are explicable by assuming the existence of enormous and confluent[172] glacier-systems in and about the high lands of Europe, Asia, and America, which high lands became, therefore, glacial radiants, and shed their load of ice in all directions over the lower adjacent ground, along the lines of easiest flow; that this theory does no violence to the analogy of the existing order of things, requiring merely an enlargement of actual glaciers by the intensification of actual conditions : that abundant evidence can be obtained, as, for example, from Switzerland, that the present glacier system of the earth was once of sufficient magnitude to produce all the observed phenomena; that the most important glacial radiants in the northern hemisphere were, in North America, the district round Hudson Bay, New England and the Adirondacks, with certain areas in the western Cordilleras, and in Europe the Norwegian Dovrefelds and the Alps, Asia apparently possessing none of commensurate importance; that it satisfactorily explains, also, the previously puzzling absence of glacial action over the great plain of Siberia, the coldest portion of the northern temperate zone; that the belief in a vast polar ice cap, thousands of feet thick, covering the whole Arctic region, and extending almost continuously down to low latitudes, is an assumption doing violence to observed physical facts and to probability, that it is not required to account for the phenomena, and is, in fact, contradictory to some of them."

[171] _American Geologist_, Feb., 1889.

[172] The term "confluent" is not necessary here. The glaciers of all mountain chains may be said to be more or less confluent in the nevé, from which individual glaciers radiate.

In Europe there is equally good evidence of the existence of huge glaciers on the Scandinavian mountains and the Alps, and of lesser accumulations of ice on the hills, as, for instance, those of the British Islands; but the Scandinavian boulders scattered over the plains of Great Britain must have been water-borne.[173]

[173] The reports of the Scottish boulder committee, and Lapworth's recent careful examination of the deposits on the East of England (_Journ. Geol. Soc._, Aug., 1891), strongly confirm me in this opinion.

In connection with these extracts I would observe that the writer, and those with whom he has acted in this matter, have never held that icebergs alone, or fields of ice alone, have produced the Pleistocene deposits. Their contention has been that the period was one in which glaciers, icebergs, and field ice acted together, and along with aqueous agencies, in producing the complicated formations of this remarkable age. They have, however, objected strenuously to the sole employment of one agent to the exclusion of others, and to attributing to that agent powers and extension which obviously could not belong to it, under the known laws which regulate the movement of glaciers by the force of gravity, and the precipitation of moisture in the form of snow on mountains and plateaus. These laws show that the movement of glaciers over level surfaces, or against the slope of the ground, and their moving stones otherwise than down slopes, are physical impossibilities, and that the accumulation of snow to form glaciers can take place only on elevated and cold land, supplied with large quantities of vapour from neighbouring water. Such accumulation can under no imaginable conditions take place in the interior plains and table lands of great continents.

Applying these laws and conclusions to the whole northern hemisphere, we learn that the conditions to produce a glacial period are the diversion of the warm currents from the northern seas, the submergence of land in the temperate regions, and its invasion by cold Arctic water, and great condensation of snow on the higher lands. Whether this condensation has a tendency finally to rectify the state of affairs, by pressing down the mountains and elevating the plains, we do not know, but I should imagine that it has not; for the high lands will, in the case supposed, be lightened by denudation, while the plains will be burdened with a great weight of deposit. Perhaps we should rather look to this as the agency for depressing and submerging the plains and elevating the hills, and suppose some other and more general pressure proceeding from the great sea basins, to effect the re-elevation of the plains.

These questions suggest that of the date of the Glacial period. This subject has recently been discussed by Prestwich and others, with the result that there is no purely geological ground for referring the Glacial age to a period so remote as that advocated by Croll on astronomical grounds. Claypole has recently discussed the matter at some length, and in a temperate spirit.[174] He takes the rate of erosion of the Niagara gorge as a measure, and shows that the Falls of St. Anthony, as described by Winchell, and all the other falls and river gorges in North America, give similar estimates, which are confirmed by the evidences of lake ridges, of the rate of erosion, and of the conditions of animal and plant life. The whole go to show that the culmination of the Glacial age may have occurred less than 10,000 years ago. He further shows that the differential elevation of Lakes Erie and Ontario, the greater ease with which the river could cut the lower part of its ravine, the probability that the part of the gorge between the whirlpool and the fall was not cut, but only cleaned out in modern times, and the possible greater flow of water in the early modern period, all tend to shorten the time required, and that, as Prestwich has inferred from other data, and the writer also in various papers, some of them of old date, the so-called post-glacial period, that of the melting away of the ice, may come within 8,000 to 10,000 years of our own time. Probably the first of these figures is the nearest to the truth,[175] so that, geologically considered, the Glacial age is very recent.

[174] _Trans. Edinburgh Geol. Soc._, vol. v., 1888.

[175] Upham, one of the ablest and most experienced of the Glacial geologists in the United States, in a recent paper on the causes of the glacial period, states similar conclusions, and adduces the evidence of Gilbert, Andrews, Wright, Emerson and others in the same sense.

Still another question of great cosmic interest relates to the possible alternation of glacial conditions in the northern and southern hemispheres. There is evidence of drift in the southern part of South America, similar to that in the north; but was it deposited at the same time? If we could be sure that it was not, many difficulties would be removed. The southern hemisphere is at present emphatically the ocean hemisphere; the northern, the land hemisphere. Perhaps these conditions may be capable of being reversed, in which case the periods of depression in the south may have corresponded with those of elevation in the north. One thing which we know is, that there is a polar ice ring, not an ice cap, for we do not know what is within its edges at the South Pole, about 2,000 miles in diameter, and this in the only circumstances in which it can exist, namely, surrounded by a vast ocean furnishing it with abundant aqueous vapour. We also know that from this ice ring radiate glaciers, carrying _débris_, with which the sea bottom is strown half way to the equator. If continents were elevated out of the Southern Ocean, we should probably have on their surfaces glacial deposits more widespread and continuous than any remaining on the continents of the northern hemisphere, and like some of them thinning out to a terminal edge or border, instead of a terminal moraine like that of a glacier.[176] Thus we may say with some truth that the southern hemisphere is now passing through one phase of the Glacial period.

[176] This is now admitted by Chamberlain and others to be the case with the oldest boulder clay on the American continent.

I have often thought that in the southern hemisphere the condition of Kerguelen Island and Heard Island, as described in the reports of the _Challenger_,[177] must very nearly represent the state of some mountain ranges and peaks in North America in the Glacial age. Heard Island, in S. latitude 53° 2′, is a mountain peak 6,000 feet high, and 25 miles in length. It sends down large glaciers to the sea. In its larger neighbour, Kerguelen, the glaciers do not reach the sea; but there is evidence that at one time they did. It is still more curious that, in Kerguelen the modern ice overlies late tertiary deposits, holding remains of large trees, indicating a more continental condition and mild climate at no very remote period. The glaciers of Heard Island and Kerguelen have, no doubt, been carrying down moraine material into the sea, and this is certainly done on a still greater scale by those of the Antarctic continent. This sends off bergs which fill the whole ocean south of 60°, and float much farther north. Some of them have been seen 2,000 feet long and 200 high, and though most of the boulders they contain are necessarily concealed, yet masses of rock, supposed to weigh many tons, have been seen on them. The whole sea bottom off this continent, as far south as 64°, consists of blue mud, with boulders and pebbles, some of them glaciated, and farther north there is, as far as 47 degrees of latitude, a considerable percentage of drift material, and this sometimes in depths of 1,950 fathoms. It is evident that, if large areas of the southern hemisphere were elevated into land, we should have phenomena to deal with not much unlike those of North America at present.

[177] Vol. i. p. 370, etc.

Perhaps no discussion carries with it more of warning to geologists to exercise caution in framing theories than this of the great ice age; and if the collapse of extreme views on this subject shall have the effect of inducing geologists to keep within the limits of well-ascertained facts and sound induction, to adhere to the Lyellian doctrine of modern causes to explain ancient phenomena, and to bear in mind that most great effects involve not one cause, but many co-operating causes, it may lead to consequences beneficial to science; and so, emerging from the cold shadows of the continental glacier, we may find ourselves in the sunshine of truth.

References:--"Acadian Geology," 1st ed., 1855; 4th ed., 1892.
Icebergs of Belle-Isle, and Glaciers of Mont Blanc, _Canadian
Naturalist_, 1865. "Notes on Pleistocene of Canada," Montreal,
1871. Papers at various dates in the _Canadian Naturalist_ and
_Canadian Record of Science_. "The Ice Age in Canada," Montreal,
1893. Canadian Pleistocene, _London Geological Magazine_, March,
1883. Flora of the Pleistocene, _Bulletin of Geological Society of
America_, vol. i., 1890, p. 311, Dawson and Penhallow.

_CAUSES OF CLIMATAL CHANGE._

DEDICATED TO

DR. T. STERRY HUNT, F.R.S.,

Whose Work in

the Chemical and Cosmical Relations of Geology

is beyond all Praise,

and is destined to command

in the Future

even greater Acceptance than in the Past.

Various Theories as to Changes of Climate--The Astronomical Theory of Croll--The Geographical Theory of Lyell--Objections of a Geological Character to the Former--Testimony of Geology and Physical Geography in Favour of the Latter