40 NEW YORK STATE MUSEUM 



possible that the two local accumulations of glacial waters just 

 referred to were parts of that general body of water. 



If, however, the lower Mohawk valley became open prior to the 

 time. when Lake Albany had developed to the extent that it covered 

 the general region from the Mohawk valley near Schenectady to the 

 Hudson, then the Mohawk waters found an outlet to the Hudson 

 channel at the lowest level left open by a topography modified by 

 glacial drift. It is possible that at this time the preglacial channel 

 of the Mohawk still afforded this passage. 



As the ice sheet receded farther to the north, uncovering the 

 region of the Glenville basin, glacio-fluviatile waters swept in con- 

 siderable volume across the floor of that depression, bearing sands 

 to the area southward. 



In the region of Burnt Hills there was a temporary halt in the 

 recession of the ice sheet, giving rise to morainic accumulations. 

 Behind these accumulations glacial waters became ponded, thus 

 originating Lake Alplaus. 



We pass then to the stage when Lake Albany had reached the 

 development evinced by the clays and sands of the plain region 

 between Schenectady and Albany. These deposits are likewise the 

 witness of a flooded condition of the Mohawk which we correlate 

 with the Iroquois stage of that river. Where the flooded river issued 

 into the lake, somewhat west of the western edge of the Schenec- 

 tady sheet, it dropped the coarser materials of its load, building a 

 bed of gravel into the lake ; the finer sediments were carried farther 

 out into Lake Albany, building a delta. For a long time these finer 

 sediments consisted mainly of clays derived from the shale rocks 

 which predominate in the drainage basin of the river. Later, when 

 the southern slopes of the i\dirondack region became freed of ice, 

 the Mohawk received from its tributaries from the north the sands 

 derived from the Precambric rocks and these were deposited in 

 Lake Albany overlying the clays. 



It was perhaps during this time that the preglacial channel of 

 the Mohawk from near Schenectady eastward was filled up by sedi- 

 ments. The alternative view is that it had been filled at an earlier 

 time by glacial debris, or drift. 



At some time after the ice in its retreat to the north had passed 

 beyond the northern edge of the area of the Schenectady sheet, the 

 waters of Lake Albany covered all parts of the area, the present 

 elevation of which is from about 350 feet in the southern part to 

 380 feet in the northern part. Besides the Mohawk delta, above 



