I am quite aware that the above sequence and the causes assumed are somewhat different from those held by many geologists with reference to regions south of Canada; but must hold that they are the only rational conclusions which can be propounded with reference to the facts observed from the parallel of 45° to the Arctic Ocean.
My own observations have been chiefly in the eastern part of North America. My son, Dr. G. M. Dawson, has much more ably and thoroughly explored those of the west; and after describing the immense Cordilleran ice mass which extended for a length of 1,200 miles along the mountains of British Columbia and discharged large glaciers to the north, as well as to the west and south, and stating his reasons for believing in that differential elevation and depression which caused the greatest height of the mountains to coincide with the greatest depression of the plains, and vice versâ, and showing the Cordilleran glacier must have been separated by a water area from that of the Laurentide hills on the east, thus concludes:—
"It is now distinctly known, as the result of work done under the auspices of the Geological Survey of Canada, and more particularly of observations by the writer and his colleagues, Messrs. McConnell and Tyrrell, that the extreme margins of the western and eastern glaciated areas of the continent barely overlap, and then only to a very limited extent, while the two great centres of dispersion were entirely distinct. For numerous reasons which cannot be here entered into, the writer does not consider it probable, or even possible, that the great confluent glacier of the north-eastern part of the continent extended at any time far into the area of the great plains; but erratics and drift derived from this ice mass did so extend, and are found between the 49th and 50th parallels, stranded on the surface of moraines produced by the large local glaciers of the Rocky Mountains. Recognising, however, the essential separateness of the western and eastern confluent ice masses, and the fact that it is no longer appropriate to designate one of these the "continental glacier," the writer ventures to propose that the eastern mer de glace may appropriately be named the great Laurentide glacier, while its western fellow is known as the "Cordilleran glacier." It may be added that there is good evidence to show that both the Laurentide and Cordilleran glaciers discharged into open water to the north."
These conclusions, based on a large induction of facts applying to a very large area of the North American Continent, coincide with my own observations in the east, and with the inferences deducible from the present condition of Greenland and Arctic America.
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.