20 NEW YORK STATE MUSEUM 



At the higher levels the clays become sandy and at the top of the 

 bluff, at a level of 340 feet, the materials are predominantly sand. 

 Transitional stages occur, where thin layers of clay, often yellowish 

 in color, are interstratified with layers of fine sand or sandy clay. 

 The materials of the bluff at the higher levels are well exposed 

 farther to the west, toward South Schenectady. In a cutting made 

 in grading the new line of the Delaware & Hudson railway, in 

 that part of the bluff at the foot of which Poentic kill flows, a 

 thickness of about 50 feet of sand's exposed. There is also a deep 

 and broad sand pit along the old line of the railroad east of South 

 Schenectady. Here the sand is for the most part coarse and at the 

 top of the pit on the south side, approaching the 360 foot level, fine 

 gravels occur. In places there are evidences that the materials were 

 laid down in turbulent waters, as shown by cross-bedding and the 

 occurrence of pockets of dark-colored, angular-grained sands. 



In regard to the mineral composition of the sands microscopic ex- 

 aminations of samples collected in several different localities showed 

 that quartz is the predominant constituent and that feldspar and a 

 dark mineral, probably magnetite, rank next in abundance. The grains 

 are generally of irregularly rounded form, though some are angular. 



It is evident from these data that, after Lake Albany had formed, 

 for a long time the Mohawk currents brought down and deposited 

 in the lake near where Schenectady is now situated great quantities 

 of clay sediments. The source of these sediments was chiefly the 

 argillaceous rocks (already largely disintegrated by glacial erosion) 

 which prevail in the drainage basin of the river. They were de- 

 posited under comparatively tranquil and constant conditions as 

 shown by the even horizontality and the fineness of the laminations 

 of the clays. This was due to the fact that the force of the currents 

 had already been largely spent, the river entering the lake some 

 miles to the west. 



At a later time sediments composed chiefly of sand were brought 

 into the lake and deposited upon the clays already laid down. These 

 sands were evidently derived from the gneissic and other crystalline 

 rocks of the Precambric formations. As the southern slopes of 

 the Adirondack region became freed from ice the streams tributary 

 to the Mohawk from the north contributed their load of sediments 

 and these coarser materials were borne to Lake Albany. The power- 

 ful currents swept the sands far out into the lake, building a broad 

 delta which eventually merged with similar accumulations formed 

 in the Hudson channel. 



With the subsidence of the waters of Lake Albany the sands be- 



