GEOLOGY OF THE PORT LeYDEN QUADRANGLE 7 



on, one layer above another, the 5^ounger deposits overlapping 

 each other and encroaching upon the sinking land surface. 

 Since the subsidence was not entirely uniform on all sides 

 certain local variations in deposition occurred. 



The first of the deposits to form upon the sinking floor was 

 the Potsdam (Cambric) sandstone now found exposed nearly 

 everywhere except along the southwest border. After this the 

 sediments changed in character and the limestones of the Beek- 

 mantown (Lower Siluric) were laid down. Then followed the 

 deposition of the highly fossiliferous Trenton (Lower Siluric) 

 limestones including the Lowville and the Black River lime- 

 stones. The fairly clear waters full of animal life then gave way 

 to the muddy waters of the Utica, when the Utica shales (Lower 

 Siluric) were deposited. At this time the Adirondack region 

 was probably all under water. Next came an uplift on the east 

 and northeast where depositions ceased. On the south and 

 southwest, however, deposition continued and the successive 

 formations of the Siluric and Devonic above the Utica shale were 

 laid down. These Paleozoic formations may now be seen as 

 one passes from the Adirondacks southward to the southern 

 border of the State. 



The last period of igneous activity in the Adirondacks 

 occurred some time after the close of the Lower Siluric. This 

 activity was of minor extent and showed itself in the form of 

 dikes. 



At some time after the deposition of the Utica shale the rocks, 

 especially along the southern border, were deformed chiefly by 

 faulting. A series of these faults extends across the Mohawk 

 valley. The western Adirondacks, including the Port Leyden 

 (juadrangle, have been subjected to erosion for a vast length of 

 time, certainly since the close of the Paleozoic and more than 

 likely since the Devonic. During this great lapse of time a large 

 amount of material has been removed. Doubtless the whole 

 Port Leyden quadrangle was at one time covered by the Utica 

 and Lorraine shales, which have all been removed except along 

 the western side. 



The superficial deposits, such as the sands and gravels which 

 are so prominent in the Port Leyden district, were formed by, 

 or along the border of, the great ice sheet of the Glacial age. 

 From the geological standpoint this ice sheet was present only 

 quite recently and covered most of New York State. 



