THE ICE AGE AND ITS WORK. 781 



THE ICE AGE AND ITS WORK. 



BY ALFKED K. WALLACE, F. E. S. 

 ERRATIC BLOCKS AND ICE-SHEETS. (Continued.') 



n. 



WE must now consider briefly the distribution of erratics in 

 North. America, because they present some peculiar fea- 

 tures and teach us much concerning the possibilities of glacier 

 motion. 



An immense area of the Northeastern States, extending south 

 to New York, and then westward in an irregular line to Cincin- 

 nati and St. Louis, is almost wholly covered with a deposit of drift 

 material, in which rocks of various sizes are imbedded, while 

 other rocks, often of enormous size, lie upon the surface. These 

 blocks have been carefully studied by the American geologists, 

 and they present us with some very interesting facts. Not only 

 are the distances from which they have been transported very 

 great, but in very many cases they are found at a greater eleva- 

 tion than the place from which they must have come. Prof. 

 G. F. Wright found an enormous accumulation of bowlders on a 

 sandstone plateau in Monroe County, Pennsylvania. Many of 

 these bowlders were granite, and must have come either from the 

 Adirondack Mountains two hundred miles to the north, or from 

 the Canadian Highlands still farther away. This accumulation 

 of bowlders was seventy or eighty feet high, and it extended 

 many miles, descending into a deep valley one thousand feet 

 below the plateau in a nearly continuous line forming part of the 

 southern moraine of the great American ice-sheet. 



On the Kentucky hills, about twelve miles south of Cincin- 

 nati, conglomerate bowlders containing pebbles of red jasper can 

 be traced to a limited outcrop of the same rock in Canada to the 

 north of Lake Huron, more than six hundred miles distant, and 

 similar bowlders have been found at intervals over the whole 

 intervening country. In both these cases the blocks must have 

 passed over intervening valleys and hills, the latter as high or 

 nearly as high as the source from whence the rocks were derived. 

 Even more remarkable are numerous bowlders of Helderberg 

 limestone on the summit of the Blue Ridge in Pennsylvania, 

 which must have been brought from ledges at least five hundred 

 feet lower than the places upon which they now lie. The Blue 

 Ridge itself shows remarkable signs of glacial abrasion, in a well- 

 defined shoulder marking the southern limit of the ice (as indi- 

 cated also by heaps of drift and erratics), so that Mr. Wright 



