336 



HISTORICAL GEOLOGY 



accumulated to considerable depths, as today in the southern 

 states. Such soils are called "residual," because they are derived 

 by the decomposition of the very rocks on which they rest. But 

 now one rarely sees rotten rock or soil in its original position well 

 within the glaciated area, because such materials were nearly all 

 scoured off by the passage of the great ice sheet, mixed with other 

 soils and ground up rock fragments, and deposited elsewhere. 

 Such are called transported soils. Along the southern side of the 



I v. v.; : J Osu/eoo sandstone 



| | Lorraine sha/e <f sandstone 



rz_^_| Utica sho/e 



Trenton //mestone 

 p^ r~b/7?e//o-Lou>v///e //mestone 

 3 Pa/eojo/c strata fconcea/ec/J 



/U\] Precam6r/c ro c/cs 



Fig. 212 

 Structure section across the Black Riv r er Valley of northern Xew York to 

 illustrate the effect of ice erosion and glacial lake deposition. Note the 

 steep front of the shale terrace which has been produced by ice erosion, 

 and the conspicuous delta deposit of the extinct glacial lake on the east 

 side. The surface of the delta deposit represents the former lake level. 

 (After W. J. MiUer, N. Y. State Mus., Bui. 135.) 



glaciated area, where the erosive power of the ice was least, rotten 

 rock is more common. Ice, shod with hard rock fragments and 

 flowing through a deep, comparatively narrow valley of soft rock, 

 is especially powerful as an erosive agent, because the tools are 

 supplied, the work to be done is easy, and the increased depth of 

 the ice where crowded into a deep, narrow valley causes greater 

 pressure on the bottom and sides of the channel. Many of the 

 valleys of northern New York were thus favorably situated for 

 ice erosion, as, for example, the Champlain, St. Lawrence, Black 

 River, Finger Lakes valleys. The writer has made a special study 

 of ice erosion in the Black River Valley of New York, and Fig. 212 



