124 NEW YORK STATE MUSEUM 



total thickness of the Potsdam is unknown, but an estimate of iooo 

 feet 1 is given. It is thickened to the northeast of the Adirondacks, 

 indicating that the Potsdam sea encroached from that direction. 

 Whether the entire Adirondacks were buried under the Potsdam 

 formation is an open question, but the writer is of the opinion that 

 they probably were, at least as far as the east-central portion is 

 concerned. 2 



The Potsdam formation has been eroded from the central mass 

 of the Adirondacks and occurs today only fringing the foothills 

 and in disconnected patches (outliers) within the borders. While 

 entirely gone from the Lake Clear region, loose boulders and irreg- 

 ular slabs are found scattered about. It is a dense, tough, white 

 to red sandstone, massively crystallized to a " quartzite." The 

 individual grains of quartz sand have been cemented together with 

 siliceous material. In the stone piles and fences of the farms the 

 occurrence of the Potsdam is surprisingly common. Some of the 

 red varieties show streaked areas of white that are attributed to 

 the bleaching action of the organic acids produced by the life in 

 the Potsdam sea. 



The Following Limestones 

 The subsidence still continuing, later beds of sedimentary rocks 

 were deposited upon the Potsdam, alternating sandstones and 

 limestones showing a thickness of from 50 to 200 feet. Following 

 these beds, which are known as the Theresa and Tribes Hill forma- 

 tions, the Little Falls magnesian limestone (dolomite) was deposited, 

 with a total thickness of some hundreds of feet. Upon these beds 

 still later formations of sedimentary rock were laid on the bottom 

 of the sea that covered nearly the whole of northeastern New York 

 State. We need not discuss these various rocks, for if they existed 

 in the area about Lake Clear, they have all been carried away by 

 erosion and the great ice sheet that invaded the land. 



Rock Structures 



Under the action of compressive forces the various minerals 

 composing the rocks of the region have been rearranged with respect 

 to one another. Those which crystallize in the form of needles or 

 s cales are forced into planes at right angles to the direction of com- 



1 H. P. Cushing, N. Y. State Mus. Bui. 95, p. 358. 



2 " Glacial Lakes and Other Glacial Features of the Central Adirondacks," 

 Amer. Geol. Soc. Bui., 27:650. 



