ANCIENT WATER LEVELS OP CHAMPLAIN-HUDSON VALLEYS 



81 



alluvial fans will form with their outer inarigin blending indefi- 

 nitelv with stream level as in the stasre numbered 0. 



So long as the ice sheet fills the valley and covers its divides, 

 terrace building within it is precluded. As soon as the ice has 

 retreated it will also have thinned, and the time will come when 

 a long tongue of ice fills the valley. Reflection of heat from the 

 bare rock walls will cause the ice to melt most rapidly on its mar- 

 gins. Along these marginal depressions with rock walls on one 

 side and ice walls on the other the drainage will escape. Tempo- 

 rary lakes may form in this situation whether the ice be in motion 

 or stagnant. The situation will then appear somewhat as follows 

 [see fig. 4]. 



Into these marginal troughs will be carried sands and gravels 

 by side streams coming from the uplands now freed from ice. 



.A.__... , A 



w ~ E 



Fig-. 4 Cross-section of valley with raai^ginal glacial deposits. Theoretic condition in 

 meridional valley back from the ice front when glacial hafe thinned in the form of a long- 

 tongue of ice or valley glacier: ^=maximum development of regional glacier ; i?=surface 

 of the ice at any given stage of thinning during retreat ; C~site of lakes and deposits on 

 margin of glacier. 



Along these depressions will also sweep the lateral streams. The 

 evident tendency will be to build deposits of gravel and sand in 

 the presence of the ice as in ordinary stream beds with a slope 

 toward sea level, but owing to melting ice with perhaps 

 sudden changes of level causing lower and lower stages 

 of gravel and sand ibuilding toward the mouths of the 

 streams. When the ice melts out, these deposits will 

 form terraces with margins reflecting more or less the form 

 of the ice sheet against which they were constructed. The effects 

 may be reproduced at successively lower levels marking stages in 

 the evacuation of the valley. These stages should be correlated 

 with frontal moraine or delta stages. Marginal remnants of the 

 ice sheet might lie out for some time to be surrounded by gravels 

 and sands, thus giving kettle holes and ice block holes in the con- 

 temporaneous ice-bound terraces, when the ice finally disappears. 



