138 ABRADING ACTION OF GLACIERS. [Ch. XI. 



Noah, while others retained the name as expressive of their opinion 

 that a series of diluvial waves raised by hurricanes and storms, or by 

 earthquakes, or by the sudden upheaval of land from the bed of the 

 sea, had swept over the continents, carrying with them vast masses 

 of mud and heavy stones, and forcing these stones over rocky sur- 

 faces so as to polish and imprint upon them long furrows and 

 striae. 



But geologists were not long in seeing that the boulder formation 

 was characteristic of high latitudes, and that on the whole the size 

 and number of erratic blocks increases as we travel toward the arctic 

 regions. They could not fail to be struck with the contrast which 

 the countries bordering the Baltic presented when compared with 

 those surrounding the Mediterranean. The multitude of travelled 

 blocks and striated rocks in the one region, and the absence of such 

 appearances in the other, were too obvious to be overlooked. Even the 

 great development of the boulder formation, with large erratics so far 

 south as the Alps, offered an exception to the general rule favorable 

 to the hypothesis that there was some intimate connection between it 

 and accumulations of snow and ice. 



Abrading, polishing, scouring and transporting power of glaciers. — 

 It is well known that those parts of the Alps which rise to heights ex- 

 ceeding 8500 feet above the level of the sea are covered with perpetual 

 snow. This snow, as it receives annual additions, would increase in- 

 definitely in altitude were not its accumulation checked by the constant 

 descent of a large portion of it by gravitation. As it glides slowly 

 down the principal valleys flanking the highest mountains, it becomes 

 converted into solid ice, and forms what are termed glaciers, or rivers 

 of ice, the lower extremities of which, when they descend into warmer 

 regions, melt and give rise to torrents of water. On the borders of 

 every glacier are seen on either side mounds, or taluses of rubbish, 

 consisting of angular fragments of rock, with large heaps of sand and 

 mud. At certain distances from each side, and often in the centre, 

 ridges composed of similar debris from three to twelve feet in height, 

 are observable. Each of these has originated, like the lateral mounds, 

 in the form of a talus accumulated at the foot of a steep slope or 

 precipice. Frost, rain, lightning, and avalanches of snow are con- 

 stantly detaching fragments of rock and soil which fall or roll down 

 to the bottom of such precipices. If the base of the heap of loose 

 materials were washed by a river, it would soon be undermined and 

 swept away, but when this fallen matter reaches the edge of a glacier, 

 which is always moving onward night and day at the rate of several 

 inches, or sometimes a foot or two in twenty-four hours, the whole 

 talus becomes locomotive, and is changed into a long stream of blocks 

 and earthy matter, fringing the glacier on both sides, and constitut- 

 ing what are called lateral moraines. As often as glaciers are conflu 

 ent, the right lateral moraine of one blends with the left moraine of 

 the other, and both are then carried down in the middle of the mass 





