GEOLOGY OF THE PORT LEYDEN QUADRANGLE 37 



Falls it is as low as 20 or 25 degrees. From south of Denley to 

 3 miles north of Lyons Falls the dip is northward; thence north- 

 ward to just south of Glenfield it is southward; thence northward 

 to the map limit it is northward. Thus we have good evidence of 

 distinct folding of the Precambric rocks on a large scale. Certain 

 other Precambric rock structures will be discussed later. 



Structure sections 



The structure sections shown in figures 2-4, page 38, have been 

 carefully chosen with the idea of giving the best general notion of 

 the various rock formations and their relationships to each other. 



PALEOZOIC OVERLAP 



It is well known that during the general subsidence (barring cer- 

 tain minor oscillations of level) of the Adirondack region in early 

 Paleozoic times, sediments were being deposited on the Precambric 

 surface and that these sediments gradually encroached upon the 

 sinking land mass until nearly all, if not all, of the Adirondack 

 region was covered by them. The younger formations extended 

 farther in than the older ones upon the sinking surface, thus con- 

 stituting an overlap of the Paleozoic sediments upon the Precambric 

 crystallines. The stripping off of this Paleozoic cover has been 

 going on since the close of the Paleozoic era, at least, and the 

 exposed surface of the Precambric rocks is still being enlarged by 

 this same process. A glance at the structure sections [fig. 2-4] 

 Vv'ill show the eroded edges of about 1500 feet of sediments which 

 formerly must have extended farther eastward upon the Pre- 

 cambric surface. In the Port Leyden district we have positive evi- 

 dence to prove the Paleozoic overlap. 



On the western side of the Adirondacks the oldest overlapping 

 formation is the Potsdam sandstone. It is not present in outcrop 

 within the Port Leyden quadrangle, the nearest exposures being 

 about 20 miles northward in the vicinity of Carthage. Accord- 

 ing to Orton 1 the Potsdam, resting upon the Precambrics, occurs 

 in Oswego county to the west and southwest of Port Leyden as 

 shown in deep well sections. Thus the deep well at Central Square 

 shows 156 feet of sandstone; the well at Parish 50 feet of sand- 

 stone and the well at Stillwater (southeast of Orwell) at least 49 



' N. Y. State Mus. Bui. 30. 



