GLACIAL GEOLOGY OF THE COHOES QUADRANGLE 21 



side of the valley, of a mile or more. These terrace plains form 

 evident and striking features of the landscape. In the field, to the 

 observer looking north and south and ignoring the depressions of 

 the gullies, the terrace surface appears as one level expanse ; or, 

 looking across the valley, as a bench broken by the ravines and 

 with the upper terrace slope and plain and the rising hills of the 

 uplands in the background. 



DEVELOPMENT OE THE LOWER TERRACE 

 The conditions under which the lower terrace was developed are 

 believed to be as follows : With the disappearance, through melt- 

 ing, of the ice lobe from the Hudson valley, the middle portion of 

 the valley became the seat of sedimentation. It had already 

 received debris derived from the melting of the ice lobe, together 

 with some accessions of finer sediments borne by the currents flow- 

 ing in the marginal channels and checked by the quiet waters of 

 the embayments lateral to the terminal portion of the ice lobe. The 

 conditions of an established body of lake waters now permitted 

 deposition from the currents normal to the lake. As the outlet of 

 the lake was at its southern end, we may assume constant south- 

 ward flowing midlake currents. These currents, moving in a body 

 of water of considerable magnitude, were of low velocity and car- 

 ried only fine sediments. Under the fluctuations incident to varying 

 seasonal, climatic and other physical factors, deposition of these 

 sediments took place and thus layers of silts and fine sands were 

 laid down on the floor of the middle portion of the lake. 



At length came the time of the subsidence of the Lake Albany 

 waters. At the first stage of subsidence the lateral portions of the 

 lake bottom had emerged as land surface, forming the upper ter- 

 races, above described. The erosion of the surfaces of these ter- 

 races immediately began and man}^ small streams, heading in the 

 uplands, extended their courses across the terraces and discharged 

 into the shrunken lake. The sediments brought by these streams 

 to the lake were distributed over the same area of the lake floor 

 that had received deposits from the midlake currents. Erom these 

 two sources were derived the silts now forming the clays of the 

 lower terrace. The emergence of these deposits, thus giving rise 

 to the present terrace, was due to a further subsidence of the Lake 

 Albany waters. 



The accompanying diagrams are believed to represent the suc- 

 cessive steps in the development of the upper and lower terraces, 

 ending with the production of the present features of the valley. 



