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NEW YORK STATE MUSEUM 



may originate in this way from terraces resulting from the re- 

 excavation of a gorge completely filled by extraglacial clays or 

 other materials after the ice has vacated the immediate district. 

 Figures 6 and 7 illustrate these conditions in their simplest mode 

 of occurrence. 



In the former case coarse gravels and sand would be expected 

 to predominate as the direct result of the outwash from the melt- 

 ing ice lying in the gorge 'but rising above the level of 

 the terraces. Clay making would go on only in lake- 

 like expansions above constrictions in the valley or down- 

 stream in the extraglacial field of that stage. The ice- 

 ward margins of such clay deposits would pass into coarser 



Fig. 6 Valley floor and gorge filled with clays 



Fig. 7 Reexcavation of gorge, showing variable erosion of clays from opposite banks 



detritus coming from the ice margin. Both sands and clays would 

 fail to be deposited in the gorge except where the ice remnant re- 

 ceded by melting from the sides of the gorge, a condition which 

 might locally occur before the ice shrank so much as to permit 

 drainage altogether through the gorge. The distinguishing char- 

 acteristic of such deposits would be evidence of contact with the 

 ice sheet along the edge of the gorge, locally coarse deposits in 

 that position, and the failure of remnants of these deposits to ap- 

 pear in the gorge except in alcoves and recesses or side channels 

 not held open by the ice. 



The distinguishing features of an excavated filling would be 

 sought in the equality of night of flood plain deposits on opposite 

 sides of the gorge, the essential identity in the lithologic charac- 

 ters of contemporaneous sections on opposite sides of the gorge, 

 and in the occurrence of remnants of the deposits in any part of 

 the gorge protected from subsequent erosion. 



