20 NEW YORK STATE MUSEUM 



tady quadrangle. The terrace surface, in these westward exten- 

 sions, shows a gradual increase in elevation. 



The general or average elevation of the upper terrace may be 

 stated as 300 feet. For the most part the 300-foot contour line of 

 the sheet marks the outer border of the terrace plain, although, in 

 places, the lacustrine deposits rise to a higher level. As a rule no 

 definite line of contact of the clays and sands with the till of the 

 uplands can be observed in the field, and in mapping this boundary 

 has been drawn somewhat arbitrarily. As pointed out by Fair- 

 child,^ the summit level of the body of glacial waters was often 

 higher than the plain of the deposits built into it. In the Lake 

 Albany waters the deltas formed at the mouths of the larger 

 streams, now represented by extensive sand plains, as that south 

 of Schenectady (Mohawk delta), north of Ballston (Hudson delta) 

 and that of the Hoosic delta, described below, probably indicate 

 closely the height of the lake waters. (Present differences in ele- 

 vation are to be accounted for by postgla'cial deformation.)^ But 

 the deposits made in Lake Albany, other than the great deltas, 

 were not usually built up to water level. Thus the plain of the 

 Hoosic delta near its head, as in the broad expanse northwest of 

 Schaghticoke, stands 340 to 360 feet elevation, while the upper 

 terrace which extends northward from the delta has an elevation 

 of 320 feet. This difference is interpreted as due to a less amount 

 of deposition taking place in the marginal channels, in the early 

 stages of the development of the lake, as above described, than at 

 the mouth of the large rivers, at the later stage when the deltas 

 were built. Also differences in elevation of the upper terrace in 

 different localities are understood as due primarily to differences 

 in amount of sedimentation. . 



The lower terrace is a quite definite topographic feature. On the 

 west side of the river it is continuous from the northern to the 

 southern margin of the sheet, except as broken by the numerous 

 ravines that cross it and where, in the localities of Mechanicville 

 and Waterford, broad stretches of the terrace were swept away by 

 the flooded Mohawk waters of late glacial times. On the east side 

 of the river the terrace shows as a distinct form feature except 

 where interrupted by the Hoosic delta and by the steep rock wall 

 of the valley at Lansingburgh. The terrace is best developed in 

 the middle part of the sheet where it attains a breadth on each 



^ Bui. Geol. Soc. of Amen, 27 :23g. 



^ Stoller. Glacial Geology of the Saratoga Quadrangle. N. Y. State 

 Mus. Bui. 183. 



