GLACIAL GEOLOGY OF THE COHOES QUADRANGLE IQ 



wall of the valley here controlling the topography. These modiii- 

 cations of the terrace form (apart from the effects of underlying 

 rock features) are clearly due in part to postglacial erosion but 

 probably in larger part to the conditions under which the materials 

 were laid down. 



DEVELOPMENT OE THE UPPER TERRACE 

 It is believed that the sands and clays of the upper terrace were 

 in large part laid dow^n at that stage of the melting of the ice when 

 the uplands had been bared, while a broad lobe of ice still lingered 

 in the Hudson valley. In the lateral depressions, between the 

 central mass of ice and the bared slopes of the valley, waters 

 gathered and flowed southward discharging into the open lake 

 waters along the dwindling southern limit of the ice lobe. These 

 currents bore sediments partly derived from the debris of the 

 melting ice and partly received from the tributary streams draining 

 the bordering uplands. The finer parts of these sediments were 

 deposited mainly where the currents were checked by the quiet 

 waters of the lower end of the marginal channel, or embayment 

 between the ice lobe and the shore of the lake. The coarser 

 materials were deposited in the bed of the channels. Also as the 

 latter shifted in position, due to the shrinking of the lobe of ice, 

 the deposits were made progressively farther inward from shore. 

 In this way the accumulations acquired the form of a shoal plat- 

 form with face sloping toward the middle of the lake. With the 

 subsidence of the lake waters, at a later time, the shoal became a 

 terrace of similar slope. (See fig. 3, page 22.) At times bodies of 

 comparatively static waters were held in portions of the lateral 

 depressions conforming to topographic features of the adjoining 

 slopes and to irregularities of the ice border. When tributary 

 streams from the upland^ discharged into these quiet waters 

 deposition took place, forming del'as. With the subsidence of the 

 waters, at a later time, these deltas emerged as terraces of more even 

 and level surfaces than those described in the preceding paragraph. 

 It is believed that an example of a terrace form developed in this 

 way is the flat area northwest of Melrose at the 300-foot level at 

 its outer border. 



On the north side of the Mohawk river and both north and south 

 of Anthony kill the plain of the upper terrace becomes continuous 

 with that of Lake Albany deposits bordering these streams and 

 extending westward to the general sand plain region of the Schenec- 



