IV THE ICE AGE AND ITS WORK 75 



in Canada to the north of Lake Huron, more than 600 

 miles distant, and similar boulders have been found at in- 

 tervals 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 boulders of Helderberg limestone 

 on the summit of the Blue Ridge in Pennsylvania, which 

 must have been brought from ledges at least 500 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 indicated also by heaps of 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 boulder transportation is, how- 

 ever, afforded by the blocks of light grey gneiss discovered 

 by Professor Hitchcock on the summit of Mount Washing- 

 ton, over 6,000 feet above sea-level, and identified with 

 Bethlehem gneiss, whose nearest outcrop is in Jefferson, 

 several miles to the north-west, and 3,000 or 4,000 feet 

 lower than Mount Washington. 



These varied phenomena of erratic blocks and rock 

 striations, together with the enormous quantity of boulder- 

 clay and glacial drift spread over the whole of the Eastern 

 States, terminating southward in a more or less abrupt 

 line of mounds having all the characteristics of an enor- 

 mous moraine, have led American geologists to certain 

 definite conclusions in which they all practically agree. It 

 may be well first to give a notion of the enormous amount 

 of the glacial debris under which a large part of the Eas- 

 tern 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 60 or 70 feet thick and sometimes more. 

 West of the Alleghanies, in New York, Pennsylvania and 

 Ohio, the thickness is much greater, being often 150 or 

 200 feet in the wide valleys and 40 or 50 feet on many 

 of the uplands. Professor Newberry calculates that in 



