68 H. L. FAIRCHILD — ICE EROSION THEORY A FALLACY 



have been comparatively stagnant and impotent. The thickness of ice 

 over Seneca valley, when the ice front was at the terminal moraine in 

 Pennsylvania, has been estimated by the writer as at least 4,000 feet.* 

 Other estimates have made the depth much greater. Certainly the press- 

 ure due to depth was very great, and it was partly resolved into a poten- 

 tial pressure down the slope, or northward, and against the general 

 movement. Three principles of ice movement were in play, namely, 

 practical viscosity, rigidity, and shearing. The problem is too compli- 

 cated to be here discussed, but the best conclusion is to the effect that 

 during the burial of the region under the general ice sheet the deep ice 

 was stagnant and erosion of the Finger Lakes valleys was impossible. 

 The stagnating effect of the bottom debris must be taken into the account 

 along with the mechanics of the clear ice. 



Valley-heads moraine. — With the waning of the ice body the front 

 receded to the line which connects the divides and moraines at the 

 present heads of the Finger Lakes valleys. There the ice front lin- 

 gered some time and accumulated the valley moraines. The front of 

 the ice was probably lobate in the valleys, and there the morainal drift 

 was concentrated ; but it should be distinctly understood that the 

 morainal masses at the heads of the Finger Lakes valleys were not 

 made by "valley" glaciers, but by lobations at the edge of the 

 great Ontarian lobe of the Labradorian ice body. When the moraine 

 was forming at the present head of Seneca valley the depth of ice over 

 the valley must have been over 2,000 feet, and the lower ice was doubt- 

 less inert. 



The moraines at the valley heads can not reasonably represent 

 erosion from the bottoms of the valleys for reasons which have been 

 given. This is an important fact which writers have overlooked, and 

 specially so since these are the only large drift accumulations which 

 the advocates of valley erosion can refer to. They are too small in vol- 

 ume and too largely composed of far-traveled material to represent much 

 valley cutting. 



Effects of ice retreat. — From the line of the valley -heads moraine to 

 the drumlin-belt filling, a distance of about 50 miles on the Seneca 

 Lake meridian, there are no morainal deposits in the valleys which have 

 been thought worthy of notice. Careful search by experts will doubtless 

 find lines of marginal drift, curving over the ridges, which will be useful 

 in proving the form and amount of lobation of the valley ice in its 



♦"Glacial geology of western New York." Geol. Mag., Dec. iv, vol. 4, no. 402, December, 1897, 

 p. 532. 



