46 NEW YORK STATE MUSEUM 



answer to this question. In many places the Potsdam has been seen 

 in contact with the underlying Prepaleozic rock whose surface 

 clearly proves that the whole region had reached a peneplain con- 

 dition. Along the northeastern side of the Adirondacks this pene- 

 plain was considerably rougher than along the northwestern, south- 

 western and southern portions. This is explained by the fact that 

 the northeastern area became submerged first and consequently 

 was not subjected to wear so long as the latter named areas. 

 In the southern Adirondack area occasional low knobs of more re- 

 sistant rock protruded above the otherwise featureless plain. 



The Potsdam sandstone formation is very easily recognizable. 

 It is almost wholly made up of clearly visible, rounded grains of 

 quartz and always well stratified or separated into relatively thin 

 layers. Ripple marks everywhere abound in the sandstone, thus 

 indicating the shallow-water (or near-shore) origin of the deposit. 

 Most of the rock is light gray to cream colored, though in some 

 places pinkish to reddish layers are common. The strata are almost 

 invariably in nearly horizontal position because they have not been 

 notably disturbed since their accumulation in the sea. All the rock 

 in the walls of the famous Ausable chasm (Clinton county) is 

 typical Potsdam sandstone. 



Marine conditions continued with the deposition of alternating 

 layers of sandstone and limestone 50 to 200 feet thick upon the 

 Potsdam sandstone. This is called the Theresa formation. After 

 still greater subsidence, an important _ formation known as the 

 Little Falls limestone (or dolomitic limestone was deposited layer 

 upon layer in the comparatively clear waters of this, the latest Cam- 

 brian sea. This formation, which is fine grained, dense, light gray 

 in color and well stratified in fairly thick beds, shows its greatest 

 thickness of 500 feet in the gorge at Little Falls in the Mohawk 

 valley. The Little Falls sea swept all around the Adirondacks 

 except what is now the southwestern border from Oneida county 

 to near the Thousand Islands. Occurrence of the formation in the 

 outlying masses at Wells (Hamilton county) and Schroon Lake 

 (Essex county) proves that the Little Falls sea extended well over 

 ■the eastern and southeastern Adirondack region. The Adirondack 

 island in this latest Cambrian sea was doubtless somewhat smaller 

 than during Potsdam time, though it was still large. Map figure 8 

 graphically illustrates the approximate relations of land and water 

 during Little Falls time. 



Ordovician period. The Cambrian period closed with the emer- 

 gence of all northern Ncav York above sea level, but very early in 



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