This section is from "The American Cyclopaedia", by George Ripley And Charles A. Dana. Also available from Amazon: The New American Cyclopędia. 16 volumes complete..
But by comparative observations made at different times, it is perceived that the great mass itself moves. The constant renewal of the waste at the lower extremity, already referred to, is one evidence of this. Objects on the surface, too, are found to be continually moving down, even when their position on the ice itself is not changed. From the high precipices at the sides masses of rock and stone fall along the edges of the glacier, but it is obvious that they do not remain there in an immovable talus; for where one glacier opens into another the piles of stones next the fork do not terminate as they join at this point, but are continued in a long mound of the same varieties of stone far down the glacier; and as other branches come in, each adds its new mound, till sometimes as many as six parallel ridges are thus produced. These may come in contact below, and thus be reduced in number, and even be blended with the piles at the edges. In some form, however, the mounds continue to the foot of the glacier; and there ridges of bowlder-shaped stones and gravel are seen, which lie in front of the glacier, and are sometimes repeated in nearly parallel lines like the little ridges of sand and drift material along a sea beach, each one of which marks the limit of some previous high tide.
So these great ridges of sand and stones, called moraines or borders, mark the limits reached by the foot of the glacier at former times; and as the tide marks are all removed when a high-course tide again sweeps far up the breach, so the ridges at certain periods are observed to move on before the advancing glacier, and mix together in a new and larger moraine at a greater distance from the mountains. It is in these periods that the habitable valleys of Switzerland are sometimes invaded by the terrible ice wall. Imperceptibly but irresistibly it is found advancing upon the farms and cottages. The warm summer weather is obviously hastening its dissolution, yet its dimensions do not sensibly diminish. The green forests slowly disappear before it; and the growing wheat almost feels its icy touch, before the soil is lifted by its ruthless ploughshare. When, after such an advance, the glacier recedes to its former bounds, the surface it covered is found to be changed into a dismal waste of loose stones.-The gathering and distribution of these materials by action of glaciers have been subjects of special interest, from the resemblance in most of the phenomena exhibited to those connected with the distribution of the geological formation known as the drift.
The loose rocks are worn into the rounded forms of bowlders, and are similarly striated and grooved upon their surface, and sometimes polished. The rocks upon and against which the glaciers have pressed are found, wherever exposed to view, to be ground smooth and deeply marked with lines corresponding in direction with the course of the glacier at the spot. It is upon these resemblances, and others connected with minor details of the two classes of phenomena, that the glacial theory of Venetz and Charpentier, so fully elaborated by Agassiz, is based, accounting for the distribution of geological formations like the drift. The transporting power of glaciers was recognized by Prof. Playfair of Edinburgh as far back as the year 1816, and the occurrence of the enormous bowlders on the Jura was attributed by him to glaciers, whose track he supposed lay at one time across the valley of Switzerland and the lake of Geneva, which now separate the Jura from the opposite summits of Mont Blanc. It is on these summits, at the distance of from 70 to 80 m., that are found the ledges of granite and other rocks, which are recognized as identical with the great bowlders scattered over the surface of the Jura limestone. (See Diluvium.)-The quantity of stony material, and the enormous size of the masses of rock carried along by glaciers, are little appreciated, even by many who have seen the loads apparently resting quietly on their surface.
Sometimes the ice is almost concealed by the accumulated piles of stone. These do not sink into the ice, except as they occasionally fall into the chasms, and even then they are sometimes brought again to the surface by the action of the forces which keep most of them there. As the rock protects the ice beneath it from the action of the sun, which has its melting effect around, the rock is thus gradually lifted upon a pedestal of ice, at the same time that the whole is slowly moving down to a lower level. When the pedestal at last gives way, the rock slips down and the process is repeated. When once in the ice, the superficial melting may bring it again to the surface. The size of the fragments is often immense. Prof. Forbes saw one in the valley which must have been brought down by the glacier, which was nearly 100 ft. long, and from 40 to 50 high; and at the foot of the glacier of Swartzburg in the valley of Saas was another estimated to contain 244,000 cubic feet, requiring an average diameter of nearly 62 ft.-The rate of progress of glaciers. dependent upon various conditions, is no more uniform than that of rivers. It can in no case be correctly estimated except by observations extending over many years.
On the glacier of Aar M. Hugi erected a hut in 1827 at the foot of a fixed and well known rock. In 183G the hut was 2,200 ft. from the rock, and in 1840 this distance had doubled. In the first period its progress had been 250 ft. per annum, and in the second 550. Forbes in 1842 found the remains of a ladder, which, it is believed, was the one left by De Saussure in 1788 at a point 16,500 ft. further up the glacier; if so, its yearly progress had been 375 ft. This movement extends through valleys in which the surface of the glacier appears to lie almost on a dead level. It is made manifest day by day by a row of stakes set up in a straight line across the glacier, and ranging with fixed points on the land at the sides. These are after a time observed to stand upon a semicircular line, the stakes near the middle moving faster than those near the margin. The importance of correctly estimating the rate of movement at short intervals and in different parts of a glacier, in order to determine the nature of the motion, appears to have been first appreciated by Agassiz in 1841, and by Forbes, who was engaged about the same time in his explorations.