GEOLOGY OF THE PENN YAN-HAMMONDSPORT QUADRANGLES 43 



On the map a thickness of ioo feet is assigned to this formation. 

 It is 65 feet thick on the Genesee river and n feet on Lake Erie. 



In the cliffs on the eastern shore of Cayuga lake, 4 miles north 

 of Ithaca, the beds of this horizon are dark to black barren shales, 

 next below the heavy sandstone that is in that locality the lowest 

 member of the Portage group and about 35 feet thick. 



The Standish flags and shales, 15 feet thick, at the base of the 

 Portage beds along Canandaigua lake and in the Middlesex valley 

 are not accessible on this quadrangle. The formation is described 

 in Museum bulletin 63. 



Middlesex black shale 



This is a well defined band of bituminous black shale overlying 

 the West River shale and exposed in many outcrops in the Middle- 

 sex valley 2 to 5 miles west of this quadrangle, and farther west to 

 Lake Erie. It is 35 feet thick in the Middlesex valley and dimin- 

 ishes to 6 feet on Lake Erie in the town of Evans, Erie co. 



There are no favorable exposures of this formation on this quad- 

 rangle. It can be recognized in a few small outcrops on the east 

 side of the Potter swamp in the vicinity of Voak, and at the mouth 

 of the Belknap gully at Guyanoga. 



The formation is fully described in Museum bulletin 63. 



Cashaqua shale 



The Cashaqua shale is a heavy mass of light bluish gray or olive 

 shales and sandstone appearing as the surface rock over a large 

 area in the north part of the Penn Yan quadrangle and outcropping 

 in the ravines and cliffs along the sides of both branches of Keuka 

 lake and as far south as 1 mile north of Hammondsport on the 

 west side and to a point opposite the village on the east side. At 

 the base in the western part of the quadrangle there are 75 feet 

 of laminated, rather sandy shales and thin flaggy sandstones that 

 are succeeded by alternating beds of heavier, more compact sand- 

 stones and soft argillaceous shales containing calcareous concre- 

 tions, and, toward the south, more or less continuous concretionary 

 calcareous layers. The entire formation becomes more arenaceous 

 and less calcareous from west to east, but the upper beds on this 

 quadrangle retain to a large degree the character and appearance 

 of the beds in this horizon exposed along Cashaqua creek in 

 Livingston county, the type locality of the formation, except that 

 the sandstones are much heavier. One of the lowest of these com- 



