28 ■ NEW YORK STATE MUSEUM 



MODIFIED TILL 



The surface materials thus far described, namely, the unmodified 

 till and the deposits made in Lakes Albaliy and Alplaus, cover about 

 five-sixths of the area of the Schenectady quadrangle. Of the re- 

 maining area much the larger part is covered by glacial till which 

 has undergone more or less extensive modifications since it was left 

 by the melting of the ice. The modifications were due either (i) 

 to additions of other materials, at first generally covering and after- 

 ward becoming more or less intermixed with the till, or (2) to the 

 removal by erosion of a portion of the materials of the till, altering 

 its surface features and to some extent its composition. 



Glacial till more or less covered and mingled with mar- 

 ginal lake deposits or with glacio-fluviatiile deposits or 

 with wind-blown sands. In the work of mapping it was found 

 that in many localities it was difficult to fix the marginal boundary 

 lines for the lacustrine deposits, since the sands of the latter graded 

 into the materials of the adjoining areas of till. This is interpreted 

 as due to marginal lake deposits having originally overlapped the 

 till and subsequently become mixed with it. Such intermixing would 

 result from the cultivation of the land, the tunneling of the soil by 

 burrowing animals and the roots of trees, and to some extent by the 

 processes of weathering. Accordingly for the sake of accuracy it 

 seemed best in the localities in question to indicate a marginal area 

 or belt separating the undoubted lake deposits from the evidently 

 unmodified till. This expedient did not, however, always render the 

 task of fixing the lines an easy one and in some localities they have 

 been drawn somewhat arbitrarily. 



A like difficulty was experienced where areas of till lie on the 

 side of the sand regions toward which the prevailing winds (north- 

 west) blow. Sands blown by the wind have been deposited on the 

 till and subsequently intermixed with it in the ways above described. 

 On the upland slopes east of Schenectady this condition is developed 

 over a considerable area. This has been mapped as modified till. 



A third condition is that produced by the spreading of materials 

 (mostly sands) over the till by glacio-fluviatile waters. The glacial 

 streams derived from the melting ice lingering on the uplands of 

 the northern part of the sheet carried the finer materials of the debris 

 derived from the glacier to lower levels. It was from this source 

 that materials deposited as sediments in Lake Alplaus were largely 

 derived. But, apparently, before the Alplaus area had become freed 



