GEOLOGY OF THE SCHROON LAKE QUADRANGLE 95 



a gradual submergence took place which allowed the sea to encroach 

 well upon the old land surface. As shown by the outliers at and 

 near Schroon Lake and elsewhere in the eastern and southern 

 Adirondacks, the first sediment to deposit on the floor of the 

 encroaching sea was the Potsdam sandstone, followed in turn by 

 the Theresa sandstone and dolomite and the Little Falls dolomite, 

 all of late Cambrian age. That the eastern and southern sides of 

 the Schroon Lake quadrangle were submerged under the late Cam- 

 brian sea is proved by the existence of the outliers of rocks of that 

 age in the Schroon valley and near North River close to the south- 

 western corner of the quadrangle. There is no evidence that the 

 late Cambrian sea covered the northwestern portion of the quad- 

 rangle (see figure 6). 



Within the quadrangle, positive data regarding the Ordovician 

 history are wholly lacking. It is however known that all of north- 

 ern New York was moderately above sea level toward the close 

 of the Cambrian, and that submergence again occurred during the 

 Ordovician so that, toward the middle of that period, all but prob- 

 ably a large low island in the east-central Adirondack region was 

 under sea water (see figure 6). It is highly probable that, during 

 part of the period at least, the Ordovician sea spread over part of 

 the Schroon Lake quadrangle. 



There is no reason to believe that marine waters ever spread 

 over any more than the fringe of the Adirondack area at any time 

 since the Ordovician. In other words, the Adirondack district 

 has been a land area subjected to erosion ever since Ordovician 

 time. 



Mesozoic and Cenozoic History 



As a result of the long time of erosion from the Ordovician 

 period to late in the Mesozoic era or early in the Cenozoic era, 

 most of northern New York was reduced to the condition of a 

 fairly good peneplain with some masses of relatively hard rocks, 

 especially in the east-central Adirondacks, rising to moderate 

 heights above the general level. 



Then the. great peneplain (commonly called the " Cretaceous 

 peneplain") was upraised and a new period of active erosion was 

 inaugurated which has continued to the present day. There is some 

 reason to think that this erosion proceeded far enough to permit 

 the larger rivers, like the Schroon, to reach an almost graded 

 condition, after which there was moderate renewed uplift. 



