Glaciation

“ Those, on the other hand, who restrict the action of land ice to local glaciers, and call in the aid of floating ice and the arctic current, maintain that the process of glaciation is limited rather by place than by time. Ever since the conditions of the earth have been such as to give rise to the formation of polar ice, the shores and the shallow seas to which the arctic current has borne it must have been subject to glacial action such as we have endeavored to describe. ”

“ Glacialists may be ranged in two schools: those who believe that one or more phases of milder climatic conditions broke up the whole Glacial period into alternating epochs of glaciation and “deglaciation”; and those who believe that the intercalated deposits represent rather the localized recessional movements of the ice-sheets within one single period of glaciation. ”

“ To cause extensive glaciation we must have, in addition to favorable eccentricity, a large precipitation. With these two factors combined we get an ice age, but not otherwise. The result has an important bearing on geologic glacial periods and their explanation.
Once formed, an ice-sheet cools everything about it and chills the climate of its hemisphere. ”

“ They stand forth defiantly as a phalanx of giant heralds proclaiming aloud the fallacy of this idea of southward glacial radiation; and in concurrence with the structure and striation of the great glacier troughs that lie between them, and the planed table-land at their summits, they establish the fact that during the greatest glaciation of the glacial epoch the ice-streams were formed on land and flowed out to sea, just as they now do at Greenland, or other parts of the world where the snow line touches or nearly approaches the level of the sea. ”

“ But evidences of more extensive mountain glaciation in the past do not in themselves prove a change in climate over the whole earth. In an epoch of fixed climate a glacier system may so deeply and thoroughly erode a mountain mass, that the former glaciers may either diminish in size or disappear altogether. ”

“ On the other hand, were all the land accumulated in the polar and temperate regions there can be little doubt that a state of almost perpetual glaciation of much of the land would result, notwithstanding that the whole earth should theoretically be at a somewhat higher temperature. ”

“ It can be demonstrated, however, that the glaciation of the Alps and of northern Europe were contemporaneous, and the facts go far to prove that the glaciers of the Rocky Mountains and the inland-ice of north-east America likewise co-existed. At all events all the old glacial accumulations of our hemisphere are of Pleistocene age, and it is for the advocates of the hypothesis under review to prove that they are not — 279 — contemporaneous. ”

“ Similar flat glaciated surfaces are met with again and again both in the Highlands and Lowlands of Scotland, occupying positions and associated with roches moutonnées and till of such a character as to prove beyond any doubt that they no less certainly are the result of the action of land-ice. But it is needless to discuss the probability or possibility of glaciation of any kind being due to floating-ice. We know that glaciers can and do polish and striate rock-surfaces ”

“ Sometimes, owing to structural peculiarities, and for various other reasons, rocks cannot resist the pressure of the ice, but are crushed and broken, and the resulting fragments are rolled and dragged forward in the direction of ice-flow. In this manner the path of a glacier becomes strewed with débris which has from time to time been forced from its rocky bed. There is really no mystery, therefore in tracking the spoor of extinct glaciers; for we have two sets of facts to aid us, either of which might suffice to indicate the extent and direction of glaciation. ”

“ We must also remember that changes of land and sea are almost always accompanied by elevation or depression of the pre-existing land: and whereas the former produces its chief effect by diverting the course of warm or cold oceanic currents, the latter is of not less importance in adding to or diminishing those areas of condensation and ice-accumulation which, as we have seen, are the most efficient agents in producing glaciation. ”

“ In both North America and Europe the high mountains, well south of the great glacier limits, especially the Sierras, Rockies, Alps, Pyrenees, and Caucasus, supported many large local glaciers in valleys which now contain none at all or only relatively small ones.
Records of glaciation, such as glacial scratches, bowlders, lakes, etc., occur high up in the White and Green Mountains, Adirondacks, Catskills, and the Berkshire Hills, thus proving that the ice must have been at least some thousands of feet thick over New England and New York. ”

“ Instances are known where glaciers have advanced over deposits of sand and gravel without disturbing them to any notable degree. Like a river, a glacier does not everywhere erode. In places it leaves its bed undisturbed and in places aggrades it by deposits of the ground moraine.
CIRQUES. Valley glaciers commonly head as we have seen, in broad amphitheaters deeply filled with snow and ice. On mountains now destitute of glaciers, but whose glaciation shows that they have supported glaciers in the past, there are found similar crescentic hollows with high, precipitous walls and glaciated floors. ”

“ So long as ocean-currents from the tropics found ready entrance to polar regions across vast tracts of what is now dry land, extreme and widespread glacial conditions were impossible. Any lowering of temperature due to cosmical causes might indeed induce new snow-fields and glaciers to appear, or existing ones to extend themselves in northern regions and the most elevated lands of lower latitudes; but such local glaciation need not have seriously affected any of the areas in which coal-seams were being formed. ”

“ It is essentially a theory of alternation; and it is certainly remarkable in how many cases geologists have independently deduced some alternations of climate as probable. Such are the interglacial deposits indicating a mild climate, both in Europe and America; an early phase of very severe glaciation when the "till" was deposited, with later less extensive glaciation when moraines were left in the valleys; several successive periods of submergence and elevation, the later ones becoming less and less in amount, as indicated by the raised beaches slightly elevated above our present coast line ”

“ These glaciations must be imagined as having occurred on a much larger scale than one might at first infer from the shrunken remnants of the glaciers that persist in the Alps and other mountains. The Pleistocene glaciers were vast sheets, hundreds of feet in thickness, sliding uniformly over valleys, hills, and mountains except for an occasional high [150] peak. ”

“ Thus the climatic conditions which caused glaciers to shrink for fifty or more years seem to have been replaced by conditions more favorable to the glaciers. Time alone will tell whether the new conditions are temporary or mark the beginning of a long cycle of wetter and cooler climate. ”

“ The northern portion of the interior of our own continent is said to be without 'distinct marks of glacial action. Should this statement be confirmed by further observation, it would not, however, be a formidable argument against a general Glacial period; for intense cold would leave no permanent record there, unless there was sufficient precipitation of moisture to form glaciers. As this region is now very dry and sterile, it was perhaps so through the Ice period, and snow at no time fell there in sufficient quantity to form glaciers. ”

“ If all that we could ever know of glacial action were confined to what we can learn from peering into the grottoes at the terminal fronts of existing glaciers, we should indeed come to the conclusion that glaciers do not erode their rocky beds to any appreciable extent. But as we do not look for the strongest evidence of fluviatile erosion at the mouth of a river, but in its valley—and mountain-tracks, so if we wish to learn what glacier-ice can accomplish, we must study in detail some wide region from which the ice has completely disappeared. ”

“ Well, many glaciers, after quitting the snow-beds by which they are fed, suddenly find themselves at the top of a precipice. Under these circumstances, as they are unable to stand still, there is but one thing for them to do, viz., go over; and as ice, though plastic, is by no means so to the extent that treacle—to which glacier ice has so often been likened—is, it is obvious that a slice will break off the advancing [Pg 88] tongue of the glacier, and come thundering down the rocks, to form material for another glacier below, or, if the quantity is insufficient, to melt gradually away. ”

“ The surface of the glacier above is not smooth and glassy like the ice of a freshly-frozen river or pond; but is white, like a field of snow. [Pg 183] This appearance is produced in part by the snow which falls upon the glacier, and in part by the melting of the surface of the ice by the sun. From this latter cause, too, the surface of the glacier is covered, in a summer's day, with streams of water, which flow, like little brooks, in long and winding channels which they themselves have worn, until at length they reach some fissure, or crevasse, into which they fall and disappear. ”

“ In such remodelled moraines, the glacier-mud has, of course, been more or less washed away. We have here a blending of the action of water with that of the glacier; and, indeed, how could it be otherwise, when the colossal glaciers of past ages gradually disappeared or retreated to the mountain-heights? The wasting ice must have occasioned immense freshets, the action of which we shall trace hereafter, when examining the formation of our drift-ponds, of our river-beds and estuaries, as well as the river-terraces standing far above the present water-level. ”

“ Glacier meadows may be seen in all stages of evolution. The lake-basin gouged by a glacier goes through many changes before it is covered by a forest and forgotten. No sooner does ice vanish and a glacier lake appear than its filling-in is commenced. Landslips and snow-slides thrust boulders and cliff-fragments into it; running water is constantly depositing sand and sediment upon its bottom. Sedge and moss commence covering its surface as soon as its water becomes shallow. In due time it becomes a bog with a thick covering like a wet mattress, composed of the matted roots of sedge and grass. ”

“ Then, the sun still cooling, the amount of evaporation fell off, till it ceased to afford the excessive supply, and the glaciers became stationary, then retrograde till they were reduced to the relatively insignificant proportions in which they now appear. This theory explains the deposition of glacial moraines in the midst of tree-ferns and a Mediterranean vegetation; for, while the glaciers were extensive in consequence of the immense precipitation everywhere but immediately upon them, a warmer climate prevailed than is now enjoyed in the same regions. ”

“ As long as the ice streams follow the torrent-channels, they act in something like the fashions of the flowing waters—to gouge out the rocks and deepen the valleys; but as the glacial period advances and the ice sheet spreads beyond the mountains enveloping the plains as well, when the glacier attains the thickness of thousands of feet, it disregards the valleys in its movements and sweeps on in majestic march across the surface of the country. As long as the continental glaciers remain the tendency is to destroy the river valleys. ”

“ The mature glacier, like the mature river, has effaced its falls and smoothed its bed to grade. It has also worn back the projecting spurs of its valley, making itself a wide channel with smooth sides. The bed of a mature glacier may form a long basin, since it abrades most in its upper and middle course, where its weight and motion are the greatest. Near the terminus, where weight and motion are the least, it erodes least, and may instead deposit a sheet of ground moraine, much as a river builds a flood plain in the same part of its course as it approaches maturity. ”

“ And on the scarred and grooved rocks was the final evidence; for these deep scratches had surely been ground by the advancing glacier as it slowly and silently moved onward. “The idea that such phenomena were not restricted to regions where glaciers now are found, but that traces of glacial action could be seen over enormous tracts of the earth’s surface, perhaps including regions in the tropics, and that in countries now temperate there might be discovered, not only the remains of tropical fauna and flora, but also distinct indications of a period of arctic cold—this was as new as startling.” ”

“ But where the climate favours abundant precipitation of aqueous vapour, on an expanse of high land amidst loftier peaks, from which steep valleys lead down to levels much below the snow-line, the glacier, fed by a perpetual growth from above, and wasted by an eternal corrosion at the lower extremity, is modified by continual transformations of interior substance, and stimulated by a never-ceasing activity of descent. ”

“ But it must be remembered that those who reject the idea of a continental glacier hold to the existence of local glaciers on the highlands more or less extensive during different portions of the great pleistocene submergence. They also believe in the extension of these glaciers seaward and partly water-borne, in the manner so well explained by Mattieu Williams; in the existence of those vast floes and fields of current-and tide-borne ice whose powers of transport and erosion we now know to be so great ”