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NEW YORK STATE MUSEUM 



tions in the sedimentary history will be greatly increased. The 

 glacial clays laid down in the outer belt of deposition of one 

 frontal stage may be eroded by the overriding action of the ice 

 of the next and then sheeted over, partly or wholly, by deposits of 

 till or boulders as well as b} 7 sheets of coarse gravel and sand. 



Another effect producing local terraces will arise during the 

 melting of ice from a gorge like that of the Hudson 

 with dissected walls quite independently of sea level so 

 long as the rock terraces rise somewhat above sea level. 

 As soon as the ice is limited to the main gorge, the 

 embayments in the wall, receiving drainage from the ice and 

 such lateral streams as may pour into them from the open 

 country, will form temporary lakes and be filled and sheeted 

 over with sands or gravels at levels determined by the effective- 

 ness of the ice barrier and the duration of the process of filling, 

 as well as by the elevation of the floor of the area of 

 deposition. 



Successive stages in the cross-section of a melting glacier in a 

 valley like, that of the Hudson river. The glacier which covered 

 eastern New York, it may be said, was pushed on to the area by 

 the pressure of its own accumulation in the Laurentide district. 

 Eliminating the effect of forward motion in the ice and supposing 

 the glacier to have been stagnant over the region between the 

 Highland canyon of the river and the Catskill mountains, it would 

 follow that for some time during the declination in the thickness of 

 the ice sheet the relations to the valley would be those indicated, in 

 figure 5, in which the ice sheet not only filled the valley but cov- 

 ered the divides on either side. 



For a long time after, when the ice had dwindled down to a 

 tongue filling the bottom of the valley, its cross-section would have 

 been that shown by BB in figure 5 and this general cross-section 

 would have been retained till a final stage was reached, when the 

 ice filled the gorge only leaving the top of the rock terraces free 

 for lateral drainage. 



In this final stage the cross-section would be that shown in DD, 

 figure 5, in which the broad rock terraces might become the seat 

 of lakes and lateral stream deposits. Upstream and behind con- 

 strictions in the valley where the terraces became wedged out as 

 in the Highlands, by unconsumed spurs from the valley sides, 



