DEPTH OF THE ICE DURING THE GLACIAL PERIOD. 195 



and Holyoke, the Berkshire Hills, and East and West Mount- 

 ain, near New Haven, were also completely enveloped in ice. 

 Between the Adirondacks and the Alleghanies the Mohawk 

 Valley was tilled nearly to the height of the Catskills, and the 

 southern edge was pushed up in Monroe, Sullivan, Tioga, 

 and Potter counties, Pa., to a height of 2,000 or 2,500 feet 

 above the sea. 



In remarking upon the accompanying sections, Professor 

 Lesley, who made them, says that while they do not satisfy 

 him in several important particulars, such as the regularity 

 of its surface, the location of possible crevasses, the descent 

 into the plain, the distribution of bowlders, etc., they serve 

 to give a correct generalized view — first, of the great thick- 

 ness of the ice-sheet, by contrasting it with the sections of the 

 solid rocks from the present surface down to the plane of 

 sea-level ; second, to allow the reader to judge for himself of 

 the extent of the eroding power at this point. We append 

 Prof essor Lesley's reasons for constructing the scale as he has : 



As to the first point, I have given to the surface of the ice 

 a gentle slope southward, by making it 600 feet thick over the 

 mountain, and 1,800 feet thick over Cherry Creek ; which 

 slope, if* continued northward, would suffice to make the ice 

 cover the highlands (2,000 feet above tide) farther north, as we 

 know that it did. Thirty years ago Agassiz gave me his law of 

 the necessary minimum thickness of a glacier for crossing a 

 barrier. It was in a conversation immediately subsequent to 

 his study of the striae on the top of Mount Desert, pointing 

 from Mount Katahdin, and descending into the sea. He said 

 that no glacier could cross a ridge unless its thickness at the 

 summit of the ridge was at least one half the height of the 

 ridge. By this rule he judged that the ice-sheet of Maine was 

 750 feet thick over the top of Mount Desert ; and this would 

 account for the great distance south of Mount Desert of the 

 terminal moraine. 



This rule was obtained by Agassiz and Desor in their long 

 residence on the glacier of the Aar, and was based on numerous 

 observations of local Alpine glaciers where they were crevassed 



