282 THE ICE AGE AND ITS WORK. 



a greater elevation than tlie i)lace from wliicli they must have come. 

 l*rof. G. F. Wright found an enonnoiivS accuniulation of bowlders on a 

 sandstone plateau in Monroe County, Pa. Many of th#se bowlders 

 were granite, and must have come either from the ^Vdirondack Moun- 

 tains, 200 miles to the north, or from the Canadian Highlands, still 

 farther away. This accunudation of bowlders was 70 or 80 feet high, 

 and it extended many miles, descending into a deep valley 1,000 feet 

 below the i)lateau in a nearly continuous line, forming ])art of the 

 southern moraine of the great American ice sheet. 



On the Kentucky hills, about lli miles south of Cincinnati, conglom- 

 erate bowlders containing" pebbles of red jasper can be tiaced to a 

 limited outcro}» of the same rock in Canada to the n(»rth of Lake 

 Huron, more tliau 000 miles distant, and similar bowldeis Ikia e 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 numereus bowlders of 

 Hclderberg limestone on the summit of the Blue Ridge, in Pennsyl- 

 vania, Avhich must have been brought from ledges at least 500 feet 

 lower than the places upon whi(;h 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 indicated also by heapsof 

 drift and erratics), so that Mr. Wright concludes that several hundred 

 feet of the ridge have been worn away by the ice. 



The crowning example of bowlder transportation is however a ffoidcd 

 by the blocks of light-gray gneiss discovered by Prof. Hitchcock on the 

 summit of Mount Washington, over 0,000 feet above sea level, and 

 identified with Bethlehem gneiss, whose nearest outcrop is in Jetter- 

 son, several miles to the northwest, and 3,000 or 4,000 feet lower than 

 Mount Washington. 



These varied phenomena of erratic blocks and rock striations, 

 together with the enormous (luantity of bowlder clay and glacial diift 

 spread over the whole of the Kastein States, terminating southward in 

 a more or less abrupt line of mounds having all the characteristics of 

 an enormous moraine, have led American geologists to certain definite 

 com-lusions in which they all practically agree. It may be well thst to 

 give a notion of the enormous amount of the glacial debris under which 

 a large part of the Eastern States is buried. In New England these 

 deposits are of less thickness than farther south, averaging from 10 to 

 20 feet over the whole area. In Pennsylvania and New York, east of 

 the Alleghanies, the deposits are very irregular, often CO or 70 feet 

 thick and sometimes more. West of the Alleghanies, in New York, 

 Pennsylvania, and Ohio, the thickness is nun-li greater, being often 150 

 or l!00teet in the wide valleys and 10 or 50 feet on many of the uplands. 

 Prof. Newberry calculates that in Ohio it averages GO feet deep over an 

 area of 25,000 square miles. 



