ANCIENT WATER LEVELS OP CHAMPLAIN-HUDSON VALLEYS 



85 



lakes might arise in which the deposition even of clay would 

 become possible. 



During the first stage when the entire district was ice covered, 

 water-laid drift would be limited to subglacial stream deposits in- 

 cluding eskers and probably some kames ; during the second stage 

 when the drainage from the top of the ice and from the valley 

 sides could escape laterally between walls 'of ice and rock, deposi- 

 tion might take place high up on the valley sides in the form of 

 lateral moraine terraces, lateral kame terraces and deltas built by 

 streams coming off the ice or down the valley sides, but the ice- 

 ward margins of these deposits would be subject to derangement 

 from the further melting of the ice. In the last stage, when the 

 ice became confined to the gorge, the rock terraces on either side 



Fig-. 5 Cross-section of dwindling ice sheet in middle Hudson valley at different stages 

 in relation to the valley form, ^^^maximum development of ice; jBB=local ice; 

 CC=ice reduced to a glacier covering old valley-floor ; DD=ice remnant filling gorge only 



would become the seat of lakes and open-air streams with a great 

 variety of deposits. Such deposits would resemble river terrace 

 deposits but near the edge of the gorge they would probably dis- 

 play coarser materials fed on to the rock terraces by the melting 

 of the ice. Kettles and kames would occur here and there where 

 depositton had taken place over and about the fringe of ice lap- 

 ping over on the rock terrace. 



The effect of any slight forward movement of the ice during the 

 progress of melting would simply tend to maintain the ice margin 

 longer at any one position and thus favor the greater development 

 of the deposits at that stage, for for-ward movement if due to sup- 

 ply of ice from behind would both thicken the ice and increase the 

 length of the glacial tongue. 



The effects of the second or valley stage offer no difficulties of 

 recognition, but in the third or gorge stage of the ice remnant it 

 is necessary to discriminate the lateral masking terraces which 



