Glaciology

“ 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. ”

“ 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. ”

“ As they creep down the ravines and gorges, blocks of rock detached by the frosts from the cliffs above, fall on the surface of the ice, and are slowly carried along with it. The bottom also of the glaciers is charged with sand, gravel, and mud, produced by the slow-crushing movement; large rocky masses become eventually worn down into fragments, and the whole surface of the hard rock below is traversed by long - 10 - parallel grooves and striæ in the direction of the glacier's course. ”

“ 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. ”

“ 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. ”

“ In consequence of the rocky walls above the sides of the glacier becoming warmed by the sun, the ice is melted near them, and hence the glacier becomes convex. Into these troughs the débris from the walls fall and form long lines of bowlders, pebbles, and sand, which are called lateral moraines. When two glaciers flow together, the two lateral moraines on the adjoining sides of each unite and form what is called a medial moraine. A third form, the terminal moraine, is the accumulation of sand and rocks which the glacier pushes before in its progress down the valley. ”

“ It is plainly one of glacier-drift. In some places he will find the various parts, such as gravel-stones, sand, clay, and bowlders, confusedly mingled, showing no assortment or stratification. The clay which has resulted from the grinding action of the glacier on the surface on which it lay, from dust blown over the ice by violent winds, as in the Alps, and from the sediment washed on the ice from the higher knobs that first became uncovered, fills all the interstices so closely as to make of the mass an impervious and uniform hard-pan. ”

“ 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. ”

“ The larger and better-defined mounds are true terminal moraines, while the — 114 — scattered heaps of rubbish point out for us the beds in which the glaciers lay. Thus, from the sea-coast up to the highest ridge of this border country, we follow the spoor of the melting ice; passing from massive and wide-spread deposits of till, gravel, and sand, and angular débris in the low-grounds, up to insignificant heaps and scatterings of rock-rubbish and angular boulders at the higher levels of the country. ”

“ 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.” ”

“ Behind projecting rocks in sheltered nooks and hollows, they accumulated, while in places exposed to the full sweep of the ice-stream they were piled up and drawn out into long parallel banks and ridges, the trend of which coincided with that of the ice-flow. The presence of confused and irregular patches and lenticular beds of sand, clay, and gravel in the till is not difficult to understand when we know that there is always more or less water flowing on underneath a glacier. ”

“ Let us now see what the fauna and flora, as far as we know it, tell us of the climate of the Glacial period. At the very outset of our inquiry we are confronted with one very serious difficulty in the problem, and that is the supposed occurrence of inter-glacial mild phases alternating with colder ones during the Ice Age. At first, when traces of a temperate flora and fauna were discovered intercalated between two layers of boulder clay, their presence was explained by the supposition of a mild inter-glacial period. ”

“ Even when bound in icy fetters, water is by no mean deprived of its active power; the glacier carries into the valley masses of its native mountain, and often, becoming ice-bergs, float with a lading of such materials far into the seas of the temperate zone. It is indisputable that vast beds of worn down fragments of the existing land are now forming into strata at the bottom of the ocean; and that many other effects are constantly produced by existing aqueous causes, which resemble some, at least, of the facts which geology has to explain. ”

“ From the wide central area descend numerous glaciers upon both the eastern and western coasts, and fimbriate by slow erosion the rockbound land. Thus carved, in the long succession of ages, deep fiords penetrate the country, walled by lofty and inaccessible precipices, and terminating at the feet of ice-tongues which protrude their burdens of rock, gravel, mud, and soil, into their waters. In places these deposits shallow the water to great distances; and far from shore, rounded rocks, transported from interior highlands, project their polished summits above the waves. ”

“ But if a great glacier fill the lower part of the valley, all the conditions of the problem are altered. Instead of the mud, sand, and stones drifted down from the higher regions being left behind in the incipient basin, they all travel onwards in the shape of moraines on the top of the ice, passing over and beyond the new depression, so that when, at the end of fifty or a thousand centuries, the glacier melts, a large and deep basin representing the difference in the movement of two adjoining mountain areas—namely, the central and the circumferential—is for the first time rendered visible. ”

“ But in these circumstances the end of the glacier not being confined laterally will spread out, and the moraine matter will be distributed over a large surface, so that the only well-marked token of its presence will be the larger masses of rock that may have been brought down. Such blocks are found abundantly in many of the districts of our own country where other marks of glaciation exist, and they often rest on ridges or hillocks over which the ice has passed, these elevations consisting sometimes of loose material and sometimes of rock different from that of which the blocks are composed. ”

“ For now that we know the vast area swept by the glacial sea, including almost the whole of central and northern Europe, and belted by land, since greatly uplifted, which then presented to the water's edge those climatal conditions for which a subarctic flora—destined to become Alpine—was specially organized, the difficulty of deriving such a flora from its parent north, and of diffusing it over the snowy hills bounding this glacial ocean, vanishes, and the presence of identical species at such distant points remains no longer a mystery. ”

“ Not only does the glacier collect a variety of foreign materials on its upper surface, but its sides as well as its lower surface are studded with boulders, stones, pebbles, sand, coarse and fine gravel, so that it forms in reality a gigantic rasp, with sides hundreds of feet deep, and a surface thousands of feet wide and many miles in length, grinding over the bottom and along the walls between which it moves, polishing, grooving, and scratching them as it passes onward. ”

“ The enormous morainic accumulations formed underneath and in front of the alpine glaciers, and over the vast areas traversed by the Scandinavian mer de glace, bear emphatic testimony to the intensity of glacial erosion. In like manner the great terraces of gravel that stretch down the valleys in front of the alpine moraines and the broad sheets of similar deposits which extend outward from the glaciated tracts of northern Europe, are equally impressive witnesses to the vigor of the flooded glacial rivers. ”

“ But when the action of glaciers was studied and the recent occurrence of a glacial epoch demonstrated as a fact, many phenomena—such as moraines and other gravel deposits, boulder clay, erratic boulders, grooved and rounded rocks, and Alpine lake basins—were seen to be due to this altogether distinct cause. There was no breach of continuity, no sudden catastrophe; the cold period came on and passed away in the most gradual manner, and its effects often passed insensibly into those produced by denudation or upheaval ”

“ After this great continental glacier passed away, or had retreated far northward, smaller and detached streams of ice still flowed southward to complete the task of moulding the valleys and lake-basins. It is to these smaller glaciers that we attribute the formation of the multitude of lakes filling rock-basins that are scattered through the northern part of the United States and over the whole of the British possessions, many of which have been hollowed out in nearly horizontal beds of rock in the same manner as lake-basins are now forming under existing glaciers. ”

“ Benefits of glaciation. Like the driftless area, the preglacial surface over which the ice advanced seems to have been well dissected after the late Tertiary uplifts, and to have been carved in many places to steep valley slopes and rugged hills. The retreating ice sheets, which left smooth plains and gently rolling country over the wide belt where glacial deposition exceeded glacial erosion, have made travel and transportation easier than they otherwise would have been. ”

“ The glacial period is not only closely related to the topography of North America and parts of Europe in the changing of river beds, the formation of lakes, the transportation of rock, the grinding down of mountains and spreading the débris over thousands of miles in extent, but it is related in an intimate way to many of the sciences, such as botany and zoölogy. A study of the flight of animals and plants in front of the great advancing ice sheet is a subject of intense interest. ”

“ 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. ”

“ ANCIENT moraines, striations denoting the action of vanished glaciers, the lost rocks, clay-beds evidently of glacial origin—all these are evidences testifying that at some period not very remote, as we count geological periods, the whole northern hemisphere down to the southern limit of 40° was submerged and covered with vast glaciers and ice-floes. There is not a living scientist of any eminence who questions the truth of this assertion. ”

“ A comparison of the effects produced by existing glaciers with certain superficial phenomena in the temperate parts of Europe and North America, renders it certain that between the Newer Pliocene and the Recent epochs, a large portion of the northern hemisphere must have been covered with a sheet of ice several thousand feet thick, like that which now envelopes the interior of Greenland. Much further south the mountains were covered with perpetual snow, and sent glaciers down every valley ”