12 NEW YORK STATE MUSEUM 



which is given below. These deposits form a mantle everywhere 

 overlying bedrock, except where ridges and masses of rock pro- 

 trude through the covering. The relief is that of a region in the 

 mature stage of erosion but with irregularities of surface some- 

 what reduced by ice abrasion and by deposits of Pleistocene and 

 recent age. 



The drainage of the uplands region is mainly through small 

 streams that flow directly to the Hudson or to the tributary rivers, 

 named above. The streams that discharge into the Hudson show 

 interesting changes in the character of their valleys as they pass 

 from the uplands areas and cross the clay and sand formation. In 

 their upper courses the streams are adjusted to the slopes of the 

 surface and to underlying rock structure and they wind between the 

 hills in open valleys, but as they debouch upon the plain of the clay 

 and sand deposits their courses become more direct and their val- 

 leys narrower and deeper, forming ravines. It is probable that in 

 their upper courses the streams occupy mainly preglacial valleys 

 while obviously the ravines have been formed in postglacial times, 

 that is, since the withdrawal of the waters in which the clays and 

 sands were deposited. 



The largest stream of the uplands region is Tomhannock creek, 

 which has its rise near the southeastern corner of the quadrangle 

 and flows northerly, emptying into the Hoosic river about 3 miles 

 from its mouth. In the upper and again in the middle part of its 

 course this stream follows broad depressions of surface which are 

 the beds of extinct lakes. In its lower course it penetrates the 

 Hoosic delta. A fuller statement of the steps of glacial history 

 recorded in these physiographic features is given later. 



DESCRIPTION AND INTERPRETATION OE THE 

 DEPOSITS 



Till or ground moraine. Till or rock debris derived from the 

 ice sheet, whether left from the bottom or deposited from the ice 

 at the time of melting, forms the mantle of materials overlying the 

 bedrock generally. In the uplands it constitutes the greater part 

 of the body of soils and subsoils but in the great valley depressions it 

 is covered by the lacustrine clays and sands, except where the latter 

 have been swept away by stream erosion. It includes all rock frag- 

 ments derived from the ice of whatever size, from boulders to grains 

 of sand and particles of clay. 



