AREA AND THICKNESS OF THE ICE-SHEET. 113 



thickness of the ice-sheet, known by the hniits of glaciation on mountains, 

 increased from a few hundred feet in the vicinity of its border to about 1 

 mile at a distance of 200 to 250 miles inside the border, both in New 

 England and in British Columbia; and from these data and from the 

 courses of glacial movement and distribution of the drift, it is computed to 

 have ranged from 1 to 2 miles or more — that is, from 5,000 to 10,000 or 

 12,000 feet — in its central portions. Probably two-thirds of a mile, or 

 about 3,500 feet, is an approximate estimate of the average thickness, 

 or, in other words, mean depth of the ice-sheet at its stage of maximum 

 development. Since in its recession the ice became the barrier of Lake 

 Agassiz, and the probable influence of its mass in producing changes in 

 the relative levels of the land and of lakes and the sea will therefore be 

 considered in a later part of this report, we may profitably review here the 

 e\"idences that it attained so g'reat extent and depth. 



Measures of the thickness of the ice-sheet and of the rates of slope 

 of its surface in New England and New York are supplied l^y its drift and 

 strife on Mount Katahdin, the White Mountains, the Green Mountains, the 

 Adirondacks, and the Catskills. 



Prof C. H. Hitchcock,^ Prof C. E. Hamlin," and others have shown 

 that the iipper limit of the drift on Mount Katahdin is about 4,700 feet 

 above the sea. The top of this mountain, which rises 600 feet higher, is 

 covered with fragments riven from the underlying rock by frost ; but it 

 appears to be destitute of drift, and probably formed an island projecting 

 out of the continental mer de glace during the stage of maximum glacia- 

 tion. If we compare the slope of the surface of the ice-sheet with the 

 present sea-level, the average ascent from the glacial border in the Atlantic 

 to Katahdin, across a probable distance of about 200 miles, was approxi- 

 mately 25 feet per mile. The greatest thickness attained by the ice upon 

 the country surrounding- the base of Katahdin was about 4,000 feet, or 

 four-fifths of a mile. 



The most noteworthy observations on the glaciation of the White 

 Mountains are those of Dr. Edward Hitchcock in 1841, marking the upper 



'Sixth Anuual Report of the Secretary of the Maine Board of Agriculture, 1861. 

 ^Bulletin of the Museum of Comparative Zoology at Harvard College, Vol. VII, 1881. 



MON XXV 8 



