224 



GLACIERS 



collection of data, concludes that the quickest rate 

 of progress of the centres of the glaciers of that 

 region averages 21 feet in twenty-four hours. In 

 many areas in Greenland , however, the limits of the 

 ice-sheets were found to be almost stationary, and 

 prolonged and careful observations became neces- 

 sary before any progress could be noted. In these 

 cases the configuration of the ground was the prin- 

 cipal cause of the more gentle motion. The varia- 

 tion in the rate of movement in different parts of 

 the mass is analogous to that of rivers, and .there 

 are many other points of similarity between 

 glaciers and streams of water which will call for 

 notice below. 



The above remarks broadly point out the general 

 movements of glaciers, but various modifying 

 agencies are frequently present, which change for 

 a time the regularity of the motion. Thus, when 

 slipping down a steep incline the rate of progress is 

 much more rapid than when level tracts or rising 

 ground are being traversed. The surface of the 

 ice-sheet, too, travels with somewhat greater 

 velocity than the lower strata, and the nature of 

 the glacier's bed here again produces modifica- 

 tions. When the path is smooth and sloping, the 

 rates of speed at which the upper and under por- 

 tions advance are much more equal than when 

 obstacles intervene, preventing the lower strata 

 from keeping up an equal ratio of motion with the 

 portions nearer to and at the surface. When the ice- 

 sheet turns aside from following a straight course 

 and forms a curve, the maximum of motion is no 

 longer in the centre, but at points along the sur- 

 face nearer to the convex side of the curve. 



In temperate and tropical latitudes the exposed 

 top of the glacier is being continually lowered and 

 reduced by evaporation, and it would appear that, 

 as a general rule, the ice masses in such situations 

 lose more by this process than they gain from the 

 snowfalls of winter. When a series or hot summers 

 and mild winters succeed each other, the amount of 

 ice dissolved and conveyed away in the form of 

 running water exceeds considerably the supply 

 brought down from higher levels by gravitation, 

 and the glacier retreats up its bed or valley. On 

 the contrary, when a succession of cold summers 

 and severe winters are experienced, it pushes itself 

 farther down, and appears, through these effects of 

 the seasons, to possess a kind of elasticity. 



When decided inequalities in the ground are 

 passed over, the hollows become filled up with ice 

 belonging to the bottom of the glacier, the super- 

 incumbent masses, passing over them ; in this 

 manner ' ice eddies ' are formed. On coming down 

 a sharp declivity the glacier becomes much cracked 

 and fissured, pinnacles and towers become con- 

 spicuous, and the whole fall presents a scene of 

 chaotic confusion. No sooner, however, is com- 

 paratively level ground again reached than the 

 pressure exerted by the flow from the heights once 

 more asserts itself, and again cakes the shattered 

 fragments into a smooth, solid whole. Crevasses 

 are cracks in the ice-sheet, at first narrow, and of 

 no great depth ; but as the glacier progresses they 

 increase in size, often assuming the dimensions of 

 huge chasms, frequently reaching from the top to 

 the bottom of the mass and travelling downwards 

 with it, until some temporary stoppage in front 

 presses the edges one against the other, and seals 

 up the orifice. 



It has been urged that, when glaciers flow over a 

 level or rising surface, something more than the 

 mere force of gravitation must be sought to account 

 for their forward movement, and the theory has 

 been advanced that water, percolating from the 

 surface through openings into the body of the ice, 

 and there undergoing expansion during the process 

 of freezing, may be a powerful factor in impelling 



the glacier onwards, where gravitation alone could 

 hardly be sufficient to account for its advance. 



Work. Glaciers have many features in common 

 with rivers. Thus, they have regular drainage 

 areas from which they draw their supplies ; they 

 move from higher to lower levels with more or less 

 rapidity as the configuration of the ground varies ; 

 the whole mass does not move at the same rate ; 

 they carry along with them rocks, boulders, gravel, 

 sand, and earth ; they reach the ocean in the forms 

 either of ice or water ; and they convey to the sea 

 their burdens of terruginous materials. Their in- 

 fluence upon marine deposits would, in the present 

 state of our knowledge, appear to be very great 

 greater, indeed, than that of the largest rivers dis- 

 charging on a bold and little indented coast, and 

 nearly as great as that of large rivers falling into 

 bays and partially enclosed seas. Thus, the con- 

 tinental marine deposits off the shores of Antarctica 

 extend almost as far out into the ocean as those 

 brought down into the Bay of Bengal and Arabian 

 Sea by the Ganges, Indus, and the other great 

 streams of India, and to an infinitely greater extent 

 than those conveyed by the great rivers of the 

 smooth, east coast of Africa, which empty them- 

 selves directly into the open ocean. 



The formation of moraines is one of the most 

 evident phenomena connected with the work of 

 glaciers. They are of three varieties, known as 

 terminal, lateral, and median. A terminal mo- 

 raine consists of a gathering of boulders, rubbish, 

 &c., pushed down by the advancing ice-sheet 

 and heaped up before it. When the glacier re- 

 treats, the moraine is seen to be of a crescent 

 shape, the extremities pointing backwards and the 

 centre pushed more or less forward evidence of the 

 greater rapidity of motion of the centre than of the 

 sides of the glacier. Lateral moraines are formed 

 by the denudation of the sides of the bed or valley 

 down which the ice-sheet flows. In its forward 

 movement it scrapes off immense quantities of 

 rubbish from the sides, which, falling on the outer 

 edges of the sheet, are carried forward and down- 

 ward and thrown off laterally. When two glaciers 

 meet, they coalesce and flow onward as one ; the 

 lateral moraines at the sides of juncture unite also, 

 and form a medial moraine down the centre of the 

 great trunk glacier. Boulders, so long as they are 

 carried upon the ice-sheets, are in nowise changed 

 by transport, preserving all their angularities and 

 sharp corners. Many of them, however, fall into 

 the crevasses, and, reaching the bottom, are ground 

 and rasped along the rocky bed of the ice-stream. 

 These boulders, as well as the solid rocks they are 

 rubbed over, become polished and striated, and in 

 this way evidence of the presence of glaciers is pre- 

 served long after they themselves have disappeared. 

 The water discharged from the extremities of ice- 

 fields is always muddy, heavily charged with a line 

 pow r der, produced by the scraping of rock and ice 

 against rock and soil. In the warmer regions, 

 when a glacier protrudes below the snow -line the 

 amount of water melted from the surface is very 

 considerable, often finding its way into a crevasse 

 and uniting with the water already collected there, 

 produced by the higher temperature prevailing in 

 the lower strata of all glaciers, and resulting from 

 the effects of pressure. The falling water in the 

 course of time drives a shaft or tunnel through 

 the ice at the bottom of the crevasse, and these 

 shafts are known as moulins. The closing of the 

 crevasse does not necessarily imply the destruction 

 of the moulin, which often remains entire, with a 

 deposit of rubbish, left by the water, all along the 

 bottom, and may come to light again through the 

 opening of a fresh chasm much farther down the 

 glacier. 



For particulars and discussions regarding glaciers and 



