GLACIATION 



GLACIERS 



223 



conain, Michigan, &c. Evidence of former exces- 

 sive glacial conditions lia-s Iwen met with in many 

 other parts of tin- world old moraines, \c. having 

 lii-fii detected in the < 'aucasus, tin- mountain- of 

 Minor, the Lelianon, tin- Himalayas, iVc. in 

 ; in the Atlas, the Kagu and Krome Moun 

 tains, &c. in Africa ; in the Andes, Tierra del 

 Fuego, &c. in South America; in New Zealand, 

 A'-. The prolwihle cause of the glacial period is 

 discussed under I'I.KI.STK:K.NK SYSTEM. 



<. laria I ion. See GLACIAL PERIOD, GLACIERS. 



are rivers of snow compacted by 

 into ice, which move slowly from higher 

 to lower levels. In tropical and temperate climates 

 glaciers are found only upon the higher parts of 

 lofty mountain-, hut at the poles whole continents 

 and great islands are entirely or partially covered 

 by them. 



Distribution. Their distribution is very exten- 

 sive : they occur in (Jreenland, which is almost 

 an entire sheet of ice ; on the islands between 

 t irceiiland and North America; in North America 

 towards the centre, in Alaska arid dotted along the 

 Pacific coast, and continued down to the extremity 

 of South America; in Europe, in Norway, among 

 the Pyrenees, and along the Alps ; in Asia they per- 

 vade the Himalayan system, and appear in Japan 

 and on the opposite mainland. The unexplored 

 Antarctic continent is, to all appearance, covered 

 entirely by one great ice-sheet of over 10,000 feet 

 in thickness. Traces of their presence in past geo- 

 logical ages are even more 

 general, appearing as they 

 do over the larger part of 

 North America, the southern 

 portion of South America, 

 all northern Europe, as well 

 as smaller areas in Africa, 

 Australia, New Zealand, 

 &c. Of the 1 155 glaciers of 

 the Alps, the longest is the 

 Aletsch, 15 miles in length ; 

 the depth of the Aar glacier 

 has lx?en estimated at 1510 

 feet. Next to the Aletsch 

 among European glaciers is 

 one in the Caucasus. 



Position. At and near 

 the equator a height of 

 16,000 feet is necessary for 

 the formation of glaciers, 

 but, as cooler regions are 

 the required 



down the Hides of the eminences by gravitation, 

 the pressure of the masse* from behind and from 

 the sides gradually hardens and compact* it, until 

 at last the air is driven out, and, the forces from 

 above acting with greater power from increase of 

 weight and inmact, the glacier assumes its liest- 

 known form that of a homogeneous concretion of 

 blue, crystalline ice. Thus slowly pushed forward, 

 the glacier continues to descend, until, in the 

 warmer latitudes, a zone is reached where the HUD 

 becomes too powerful to be resisted, and the ice 

 melts, thus forming the headwaters of rivers, many 

 of which take their origin in this way. In more 

 rigorous climates the ice-sheets are pushed down to 

 the lowest-lying grounds, until their edges are pro- 

 truded into the sea, and until a sufficient depth of 

 water is reached to Moat the buoyant ice, which is 

 now submerged to two-thirds of its thickness. 

 Partly by the action of the swell, partly Itecause 

 of its own weight, the edge becomes detached from 

 the parent mass, and floats out to sea in the form 

 of Icebergs (q.v. ). This process of dissolution is 

 known among whalers as 'calving.' But even in 

 the higher latitudes, such as Greenland, where the 

 temperature is always exceedingly low, the ice 

 dissolves and reaches the sea by rivers as well &n 

 by icebergs. The melting in such cases is alm<>-t 

 entirely due to pressure, the water escaping from 

 below the ice-sheet. The solar influences being 

 weak, even in the height of summer the supply of 

 moisture derived from the exposed surfaces in these 

 regions is small and insignificant. 



altitude Incomes less and 

 less, until the poles are 

 reached, where the ice-sheets are presented empty- 

 ing themselves into the ocean. But wherever 

 occurring, they are always greatest and most fre- 

 quent on eminences of the required height, which 

 first meet the vapour-laden winds coming from the 

 sea, and presenting a side or sides out little 

 exposed to solar influences. Thus, the Himalaya 

 Mountains, being directly in the track of the 

 south-west monsoon, with no intervening heights 

 of any consequence between them and the ocean, 

 first receive its watery burden, with the conse- 

 quent formation of the great glaciers of that 

 region. In the same way the Andes of South 

 America, meeting the breezes from the Pacific, 

 bear great ice-sheets upon all their more prominent 

 peaks. In New Zealand, while the glaciers of the 

 Mount Cook range reach down to 700 feet above 

 the sea on the west side, they reach only to 2000 

 feet on the east side. 



Movement. On the higher summits of glacier- 

 bearing mountains the snow lies loose, in granular 

 form and comparatively lightly ; but, as it is impelled 



The edge of the Muir Glacier, Alaska 



Although the onward movement of a glacier is 

 too slow to be perceptible to the eye, it is none 

 the less present and, generally, continuous. J. 

 D. Forbes found ffrom measurements made by 

 himself in the Mer de Glace, near Chamouni ; 

 see ALPS) and first proved that the whole 

 sheet does not possess the same rate of motion, 

 the centre advancing more rapidly than the 

 sides. He discovered that in summer and in the 

 fall of the year the middle of that glacier drew 

 forward at a rate of from 1 foot 8 inches to 2 feet 

 3 inches, and at the sides at from 1 foot 1 inch 

 to 1 foot 7 inches per diem. Agassiz at al*>ut 

 the same time carried on a series of independent 

 experiments on the glacier of the Aar, and arrived 

 at similar conclusions. Helland later on demon- 

 strated that in Greenland a more rapid motion 

 was to be found, and that the Jacolwhafn glacier 

 advanced at a rate of from 48-2 feet to 64 '8 

 feet in the twenty-four hours. This result has 

 lately been generally confirmed, although somewhat 

 modified, by Dr itink, who. from a considerable 



