28 



NEW YORK STATp MUSEUM 



this valley and discharging into waters flowing southward in the 

 Hudson valley. The east terrace likewise has the location and 

 form relations which would result from currents from the same 

 tributary river sweeping across the Hudson valley waters and 

 impinging against a frontal lobe of the delta mass. 



300ft 



200 ft. 



loott. — 



u 



.0) 



-4-> 



(D 

 u 



o 



4-> 



-4-» 



0) 



^ 



O 





CO 



^ 



o 



p- 



o 



^- 



^ 



J 



UJ 



' ' 











'■^.^^ 







■V 



' 





' 



Fig. 6 Surface profile across the Hudson valley at Mechanicville showing 

 the symmetrical erosion ' terraces. Horizontal scale and relief scale cor- 

 respond to those of topographic sheet. The line on which the profile is 

 drawn is shown on the map. 



These terraces arc therefore interpreted as follows : When the 

 waters of Lake Albany had so far subsided as to bring to the 

 surface the deposits which now form the lower, or brick-clay, ter- 

 race of the Hudson valley, the Iroquois-Mohawk waters still dis- 

 charged into the lake through the northern (Anthony kill) channel. 

 In the progress of the subsidence the river currents, shifting east- 

 ward, cut down the inner portion of the clay terrace at Mechanic- 

 ville and, due to the deflection of the Iroquois-Mohawk currents, 

 southward through confluence with the southward-flowing lake 

 currents, the area of reduced clay surface developed southeasterly. 

 This process of erosion was brought to an end by a renewed and 

 more rapid subsidence of the lake waters, thus causing the eroded 

 area to emerge as land surface and forming the present terrace. 



This terrace and the corresponding one on the opposite side 

 of the valley are therefore features due to the destructive (degrad- 

 ing) work of river currents in contrast with the clay terraces which 

 were built up through the deposition of sediments in Lake Albany. 



