260 THE ICE AGE IN NORTH AMERICA. 



feet. In this vicinity, Professor I. C. White * reports wells 

 fifty feet deep which barely reach through the till, and this 

 on elevations 1,335 feet above tide. 



Great as these amounts may seem, the estimation of the 

 erosion in the Scandinavian Peninsula by Professor Helland 

 is still larger, f Helland, after conference with several 

 geologists familiar with the region, estimates that the aver- 

 age depth of the drift over north Germany and northwest 

 Kussia is 150 German feet. This would indicate that the 

 erosion from the Scandinavian Peninsula had been as much 

 as 250 feet, since the material has nearly all been derived 

 from Scandinavia, and the area of the source of supply in 

 Scandinavia is only two fifths of that over which it was dis- 

 tributed. 



This estimate of Helland for Scandinavia is not, how- 

 ever, greater than that of Professor Lewis for northern 

 Pennsylvania. Here, on the Kittatinny Mountain, near the 

 Delaware Water-Gap, this observer seems to have had a 

 rare opportunity for directly measuring the eroding power 

 of the ice at that point.J The summit of the mountain is 

 crowned by compact strata of Medina sandstone, and trends 

 northeast by southwest. The glacier surmounted the ridge 

 on both sides of the Water-Gap, and extended twelve or 

 fifteen miles farther south, while to the southwest the sum- 

 mit of the mountain was outside of the line of ice-movement, 

 which just here is bordered by cliffs of the crowning Medina 

 sandstone seventy feet high, as if the ice, in moving past 

 them, had worn down the strata underneath to that amount. 

 Professor Lesley, at the Minneapolis meeting of the Ameri- 

 can Association for the Advancement of Science, in 1884, 

 took this as a measure of glacial erosion to illustrate how 



* See " Second Geological Survey of Pennsylvania on Wyoming, Lackawanna, 

 Luzerne, Columbia, Montour, and Northumberland Counties G 7 ," p. xiii. 



| " Ueber die Glacialen Bildungen der nordeuropaischen Ebene, Deutsch. 

 Geol. Gesell.," Zft. xxxi, 1879, p. 97. 



+ "Second Geological Survey of Pennsylvania Z," pp. 70, 90. See also 

 above, p. 196. 



