J. W. Dawson on the Glacial Theory
From: Acadian Geology, 4th Edition. MacMillan & Co., London, 1891. Chapter V, pp. 66-73.
Origin of Drift
I have retained the above explanation of the boulder-clay, which appeared in my edition of 1855, because I have as yet seen no reason to change my opinion on the subject, although I have since that time had opportunities of studying the Post-pliocene of Canada and other parts of America and Europe, and have read nearly all that has been written by the advocates of a terrestrial origin of this deposit, in a supposed glacial period when the whole of the northern parts of Europe and America are imagined to have been covered with glaciers, or rather with a universal glacier like that of Greenland, but on an enormously larger scale. The more I have considered this hypothesis, the more improbable it has appeared, whether in a mechanical, meteorological, or geological point of view; and a recent visit to Mont Blanc, and the study of the effects produced by icebergs in the straits of Belleisle, have more fully established in my mind the belief that floating ice and the Arctic current have been the grand agents employed. As the glacier hypothesis of Agassiz, Ramsay, and others, has been incorporated into the best American text-book of geology, that of Professor Dana, and has recently been ably advocated in the case of New Brunswick, I may here give some of my reasons for dissenting from it, as stated in a paper published some time ago in Canada.
The facts to be accounted for are the striation and polishing of rock surfaces, the deposit of a sheet of unstratified clay and stones, the transport of boulders from distant sites lying to the northward, and the deposit on the boulder clay of beds of stratified clay and sand, containing marine shells. The rival theories in discussion are, first, that which supposes a gradual subsidence and re-elevation, with the action of the sea and its currents, bearing ice at certain seasons of the year; and, secondly, that which supposes the American land to have been covered with a sheet of glacier several thousands of feet thick.
The last of these theories, without attempting to undervalue its application to such regions as those of the Alps or of Spitzbergen or Greenland, has appeared to me inapplicable to the drift deposits of eastern America, for the following reasons:
1. It requires a series of suppositions unlikely in themselves and not warrented by facts. The most important of these is the coincidence of a wide-spread continent and a universal covering of ice in a temperate lattitude. In the existing state of the world, it is well known that the ordinary conditions required by glaciers in temperate lattitudes are elevated chains and peaks extending above the snow-line; and that cases in which, in such latitudes, glaciers extend nearly to the sea-level, occur only where the mean temperature is reduced by cold ocean-currents approaching to high land, as for instance in the Tierra del Fuego and the southern extremity of South America. But the temperate region of North America could not be covered with a permanent mantle of ice under the existing conditions of solar radiation; for, even if the whole were elevated into a table-land, its breadth would secure a sufficient summer heat to melt away the ice, except from high mountain-peaks. Either, then, there must have been immense mountain-chains which have disappeared, or there must have been some unexampled astronomical cause of refrigeration, as, for example, the earth passing into a colder portion of space, or the amount of solar heat being diminished. But the former supposition has no warrant from geology, and astronomy affords no evidence for the latter view, which, besides, would imply a dimunition of evaporation militating as much against the glacier theory as would an excess of heat. An attempt has recently been made by Professor Frankland to account for such a state of things by the supposition of a higher temperature of the sea, along with a colder temperature of the land; but this inversion of the usual state of things is unwarrented by the doctrine of the secular cooling of the earth; it is contradicted by the fossils of the period, which show that the seas were colder than at present; and if it existed, it could not produce the effects required, unless a preternatural arrest were at the same time laid on the winds, which spread the temperature of the sea over the land. The alleged facts observed in Norway, and stated to support this view, are evidently nothing but the results ordinarily observed in ranges of hills, one side of which fronts cold sea-water, and the other land warmed in summer by the sun.
The supposed effects of the varying eccentricity of the earth's orbit, so ably expounded by Mr. Croll, are no doubt deserving of consideration in this connexion; but I agree with Sir Charles Lyell in regarding them as insufficient to produce any effect so great as that refrigeration supposed by the theory now before us, even if aided by what Sir Charles truly regards as a more important cause of cold, namely, a different distribution of land and water, in such a manner as to give a great excess of land in high latitudes.
2. It seems physically impossible that a sheet of ice, such as that supposed, could move over an uneven surface, striating it in directions uniform over vast areas, and often different from the present inclination of the surface. Glacier ice may move on very slight slopes, but it must follow these; and the only result of the immense accumulation of ice supposed, would be to prevent motion altogether by the want of slope or the counteraction of opposing slopes, or to induce a slight and irregular motion toward the margins or outward from the more prominent protuberances.
It is to be observed, also, that, as Hopkins has shown, it is only the sliding motion of glaciers that can polish or erode surfaces, and that any internal changes resulting from the mere weight of a thick ice sheet resting on a level surface, could have little or no influence in this way.
3. The transport of boulders to great distances, and the lodgement of them on hill-tops, could not have been occasioned by glaciers. These carry downward the blocks that fall on them from wasting cliffs. But the universal glacier supposed could have no such cliffs from which to collect; and it must have carried boulders for hundreds of miles, and left them on points as high as those they were taken from. On the Montreal Mountain, at a height of 600 feet above the sea, are huge boulders of feldspar from the Laurentian Hills, which must have been carried 50 to 100 miles from points of scarcely greater elevation, and over a valley in which the striae are in a direction nearly at right angles with that of the probable driftage of the boulders. Quite as striking examples occur in many parts of this country. It is also to be observed that boulders, often of large size, occur scattered through the marine stratified clays and sands containing sea-shells; and whatever views may be entertained as to other boulders, it cannot be denied that these have been borne by floating ice. Nor is it true, as has been often affirmed, that the boulder clay is destitute of marine fossils. At Isle Verte, Riviere du Loup, Murray Bay, and St Nicholas on the St Lawrence, and also at Cape Elizabeth, near Portland, there are tough stony clays of the nature of true "till," and in the lower part of the drift, which contain numerous marine shells of the usual Post-pliocene species.
4. The Post-pliocene deposits of Canada, in their fossil remains and general character, indicate a gradual elevation from a state of depression, which on the evidence of fossils must have extended to at least 500 feet, and on that of far-travelled boulders to several times that amount; while there is nothing but the boulder clay to represent the supposed previous ice-clad state of the land, except the scratches on the rock surfaces, which must have been caused by the same agency which deposited the boulder clay.
5. The peat deposits, with fir-roots, found below the boulder clay in Cape Breton, the remains pf plants and land-snails in the marine clays of the Ottawa, and the shells of the St Lawrence clays and sands, show that the sea at the period in question had nearly the temperature of the present Arctic currents of our coasts, and that the land was not covered with ice, but supported a vegetation similar to that of Labrador and the north shore of the St Lawrence at present. This evidence refers not to the later period of the Mammoth and Mastodon, when the re-elevation was perhaps nearly complete, but to the earlier period contemporaneous with or immediately following the supposed glacier period. In my former papers on the Post-pliocene of the St Lawrence, I have shown that the change of climate involved is not greater than that which may have been due to the subsidance of land, and to the change of course of the Arctic current, actually proved by the deposits themselves.
These objections might be pursued to much greater length; but enough has been said to show that there are, in the case of north-eastern America, strong reasons against the existence of any such period of extreme glaciation as supposed by many geologists; and that if we can otherwise explain the rock striation and polishing, and the formation of fiords and lake basins, the strong points with these theorists, we can dispense altogether with the portentious changes in physical geography involved in their views, and which are not necessary to explain any of the other phenomena.
On these points, the Report of the Geological Survey of Canada throws new light; though Sir William Logan, with his usual caution, has not committed himself to theoretical conclusions; and in one or two local cases he seems to favour the glacial theory. It has long been known to geologists, that in north-eastern America two main directions of striation of rock surfaces occur, from north-east to south-west, and from north-west to south-east; and that locally the directions vary from these to north and south and east to west. Various attempts have been made, but without much success, to account for these directions of striation by the motion of glaciers; and while it is quite easy for any one prepossessed with this view to account in this way for the striation in a particular valley or part of a valley, yet so many exceptional facts occur as to throw doubt on the explanation, except in the case of a few of the smaller and steeper mountain gorges.
In the Report of the Survey of Canada a valuable table of these striations is given, from which it appears that they are locally distributed in such a way as to throw a decided gleam of light on their origin.
It would seem that the dominant direction in the valley of the St Lawrence, along the high lands to the north of it, and across western New York, is north-east and south-west; and that there is another series of scratches running nearly at right angles to the former, across the neck of land between Georgian Bay and Lake Ontario, down the valley of the Ottawa, and across parts of the Eastern Townships, connecting with the prevalent south and south-east striation which occurs in the valleys of the Connecticut and Lake Champlain, and eslewhere in New England, as well as in Nova Scotia and New Brunswick. What were the determining conditions of these two courses, and were they contemporaneous or distinct in time? The first point to be settled in answering these questions is the direction of the force which caused the striae. Now, I have no hesitation in asserting, from my own observations as well as from those of others, that for the south-west striation the direction was from the ocean toward the interior, against the slope of the St Lawrence valley. The crag-and-tail forms of all our isolated hills, and the direction of transport of boulders carried from them, show that throughout Canada the movement was from north-east to southwest. This at once disposes of the glacier-theory for the prevailing set of striae; for we cannot suppose a glacier moving from the Atlantic up into the interior. On the other hand, it is eminently favourable to the idea of ocean drift. A subsidence of America, such as would at present convert all the plains of Canada and New York and New England into sea, would determine the course if the Arctic current over this submerged land from north-east to south-west; and as the current would move up a slope, the ice which it bore would tend to ground, and to grind the bottom as it passed into shallower water; for it must be observed that the character of slope which enables a glacier to grind the surface may prevent ice borne by a current from doing so, and vice versa.
Now we know that in the Post-pliocene period eastern America was submerged, and consequently the striation at once comes into harmony with other geological facts. We have, of course, to suppose that the striation took place during submergence, and that the process was slow and gradual, beginning near the sea and at the lower levels, and carried upwards to the higher grounds in successive centuries, while the portions previously striated were covered with deposits swept down from the sinking land or dropped from melting ice. It would be easy to show that this view corresponds with many of the minor facts.
Further, the theory thus stated accounts for the excavation of the deep and land-locked basins of our great American lakes. Ocean currents, if cold, and clinging to the bottom.must cut out pot-holes, just as rivers do, though geologists are too apt to limit their function to the throwing up of banks. The course of the present Arctic current along the American coast has its deep hollows as well as its sand- banks. Our American lake-basins are cut out deeply into the softer strata. Running water on the land would not have done this, for it could have no outlet; nor could this result be effected by breakers. Glaciers could not have effected it; for even if the climatal conditions for these were admitted, there is no height of land to give them momentum. But if we suppose the land submerged so that the Arctic current, flowing from the north-east, should pour over the Laurentian rocks on the north side of Lake Superior and Lake Huron, it would necessarily cut out of the softer Silurian strata just such basins, drifting their materials to the south-west. At the same time, the lower strata of the current would be powerfully determined through the strait between the Adirondac and Laurentide Hills, and, flowing over the ridge of hard rock which connects them at the Thousand Islands, would cut out the long basin of Lake Ontario, heaping up at the same time, in the lee of the Lawrentian ridge, the great mass of boulder clay which intervenes between Lake Ontario and Georgian Bay. Lake Erie may have been cut by the flow of the upper layers of water over the Middle Silurian escarpment; and Lake Michigan, though less closely connected with the direction of the current, is, like the others, due to the action of a continuous eroding force on rocks of unequal hardness.
The predominant south-west striations, and the cutting of the upper lakes, demand an outlet to the west for the Arctic current. But both during depression and elevation of the land, there must have been a time when this outlet was obstructed, and when the lower levels of New York, New England, and Canada were still under water. Then the valley of the Ottawa, that of the Mohawk, and the low country between Lakes Ontario and Huron, and the valleys of Lake Champlain and the Connecticut, would be straits or arms of the sea, and the current, obstructed in its direct flow, would set principally along these, and act on the rocks in the north and south and north-west and south-east directions. To this portion of the process I would attribute the north-west and south-east striation. It is true that this view does not account for the south-east striae observed on some high peaks in New England; but it must be observed that even at the time of greatest depression, the Arctic current would cling to the northern land, or be thrown so rapidly to the west that its direct action might not reach such summits.
Nor would I exclude altogether the action of glaciers in eastern America, though I must dissent from any view which would assign to them the principal agency in our glacial phenomena. Under a condition of the continent in which only its higher peaks were above the water, the air would be so moist, and the temperature so low, that permament ice may have clung about mountains in the temperate lattitudes. The striation itself shows that there must have been extensive glaciers, as now, in the extreme Arctic regions. Yet I think that most of the alleged instances must be founded on error, and that old sea-beaches have been mistaken for moraines. I have observed in Canada and Nova Scotia many old sea-beaches, gravel-ridges, and lake-margins, I have seen nothing that could fairly be regarded as the work of glaciers. The so-called moraines, in so far as my observation extends, are more probably shingle beaches and bars, old coast-lines loaded with boulders, trains of boulders or "ozars." Most of them convey to my mind the impression of ice-action along a slowly subsiding coast, forming successive deposits of stones in the shallow water, and burying them in clay and smaller stones as the depth increased. These deposits were again modified during emergence, when the old ridges were sometimes bared by denudation, and new ones heaped up.
I shall close these remarks, perhaps already too tedious, by a mere reference to the alleged prevalence of lake basins and fiords in high northern latitudes, as connected with glacial action. In reasoning on this, it seems to be overlooked that the prevalence of hard metamorphic rocks over wide areas in the north is one element in the matter. Again, cold Arctic currents are the cutters of basins not the warm surface-currents. Further, the fiords on coasts like the deep lateral valleys of mountains are evidences of the action of the waves and currents rather than that of ice, I am sure that this is the case with the numerous indentations of the coast of Nova Scotia, which are cut into the softer and more shattered bands of rock; and show, in raised beaches and gravel ridges like those of the present coast, the levels of the sea at the time of their formation.
In Nova Scotia we have the means of applying another crucial test to this theory of lake basins. The whole surface of the peninsula has been striated and polished; and it has been estimated that one-third of its area is occupied by lakes, most of them of small dimension. Now these lakes are almost entirely confined to those metamorphic regions where unequal hardness and imperfect facilities for drainage tend to promote their occurrence. It is evident, therefore, that we are to seek for the origin of the lake basins in these local causes, and not in any universal covering of glacier. Further, as I have above shown, the manner in which the great Canadian lakes have been cut out of the softer materials, and their relations to the neighboring harder portions of the country, prove that these great basins may be due to the action of ocean-currents, a cause to which I would attribute also the greater part of the smaller lakes of Nova Scotia. To these reasons I may add the following comparative statements of the effects of glaciers and icebergs, deduced from examinations of the glaciers of Mont Blanc and the icebergs of Belleisle:
1. Glaciers heap up their debris in abrupt ridges. Floating ice sometimes does this, but more usually spreads its load in a more or less uniform sheet.
2. The material of moraines is all local. Icebergs carry their deposits often to great distances from their sources.
3. The stones carried by glaciers are mostly angular, except where they have been acted on by torrents. Those moved by floating ice are more often rounded, being acted on by the waves and by the abrading action of sand drifted by currents.
4. In the marine glacial deposits mud is mixed with stones and boulders. In the case of land glaciers most of this mud is carried off by streams and deposited elsewhere.
5. The deposits of floating ice may contain marine shells. Those of glaciers cannot, except where, as in Greenland and Spitzbergen, glaciers push their moraines out into the sea.
6. It is of the nature of glaciers to flow in the deepest ravines they can find, and such ravines drain the ice of extensive areas of mountain land. Icebergs, on the contrary, act with greatest ease on flat surfaces or slight elevations in the sea bottom.
7. Glaciers must descend slopes and must be backed by large supplies of perennial snow. Icebergs act independently, and, being water-borne, may work up slopes and on level surfaces.
8. Glaciers striate the sides and bottoms of their ravives very unequally, acting with great force and effect only on those places where their weight impinges most heavily. Icebergs, on the contrary, being carried by constant currents and over comparatively flat surfaces, must striate and grind more regularly over large areas, and with less reference to local inequalities of surface.
9. The direction of the striae and grooves produced by glaciers depends on the direction of valleys. That of icebergs, on the contrary, depends upon the direction ofmarine currents, which is not determined by the outline of the surface, but is influenced by the large and wide depressions of the sea-bottom.
10. When subsidence of the land is in progress, floating ice may carry boulders from lower to higher levels. Glaciers cannot do this under any circumstances, though in their progress they may leave blocks perched on the tops of peaks and ridges.
Related LinksSir J. William Dawson - Nova Scotia Museum site