198 W. UPHAM PLEISTOCENE AND PRESENT ICE-SHEETS. 



the Muir glacier was about 14 inches per week, which would lower its 

 surface probably 15 or 20 feet in the whole season. This corresponds 

 approximately with the ablation of the Mer de Glace in Switzerland, 

 ascertained by Forbes to be 241 feet between June and September in 

 1842, while in some exceptional cases the ablation of glaciers in summer 

 has been found to be as much as one foot a day. 



One other point of great significance brought out by these investigations 

 of the Muir glacier is the approximate determination of the rate of gla- 

 cial erosion upon its rock bed. Measurements of the sediment in the 

 water of the copious subglacial streams, and estimates of the quantity of 

 water annually discharged from the rainfall and snowfall of the Muir 

 basin, indicate, according to Wright's computations, an average erosion 

 of one-third of an inch for all the ice-clad area in a year, while according 

 to Reid it would be about three-fourths of an inch. 



Inferences from Comparisons of present and Pleistocene Ice- 

 sheets. 



Probable surface Slopes and Tliickness of the Pleistocene Ice-sheets of North 

 America and Europe. — In North America the upper limits of the Pleisto- 

 cene glaciation on mount Katahdin, the Catskills, the Three buttes or 

 Sweet Grass hills of Montana, the Rocky mountains north of the inter- 

 national boundary, and the mountains of British Columbia, give us reli- 

 able information of the thickness of the ice-sheet in the vicinity of these 

 high elevations of land. Its depth is computed by Dana to have been 

 about two miles on the Laurentide highlands, between the Saint Law- 

 rence and Hudson bay, whence the ice flowed radially outward in all 

 directions, during its maximum stage overtopping the White, Green and 

 Adirondack mountains. In British Columbia, according to Dr G. M. 

 Dawson, its maximum depth was about 7,000 feet. These thicknesses, 

 however, which seem well determined, would not give to the borders of 

 the North American ice-sheet surface slopes of more than about 25 to 30 

 feet per mile, whereas the Greenland ice-sheet is known to have surface 

 gradients of 100 to 200 feet per mile. Apparently slopes of at least 50 

 feet or more per mile for the outer portion of the ice-sheet are required 

 to produce strong glacial currents, such as transported bowlders 1,000 

 miles, from the eastern side of the southern part of Hudson bay where it 

 narrows into James bay south westward to southern Minnesota, and such 

 as carried Scandinavian bowlders likewise about 1,000 miles, from their 

 sources to central Russia. The Pleistocene ice-sheets could have had 

 gradients comparable in steepness with those of the Greenland ice-sheet 

 only by epeirogenic uplifts of the central portions of their areas to heights 



