I-I^ 



iO 1\^EW YOilK STATE MUSEUM 



sive limestone^ quite pure but containing lenses of cllert, wliicb 

 are cliiefly abundant in the lower beds. At times it disappears 

 altogether, but this is not usual. One of the best ledges of Onon- 

 daga limestone is in the cliffs near Oniskethau creek at Clarks- 

 ville. 



Lotuer Helderherg limestone. This reaches a large dcA^elopment 

 in Albany county and is divisible into several well marked mem- 

 bers. The foremost of these is the Becraft, also, known as the 

 Scutella limestone. This rock is of light color, often crystalline 

 and full of fossils. Its average thickness in Albany county is 

 about 15 feet, and its composition may be inferred from an analy- 

 sis given of the same bed occurring at Eondout, Ulster co., and 

 Hudson, Columbia co. One exposure of it is in the creek bed 

 south of Callanans Corners. Underlying the Becraft limestone 

 is a series of diiierent beds, of very impure, highly fossiliferous^ 

 shaly limestone of a gray and grayish brown color and probably too 

 impure for any use except building or road-making. Their thickness 

 averages 100 feet. Under these, however, comes the Pentamerus 

 limestone, which is an important member of the Helderberg for- 

 mation, whose outcrop is marked by lines of prominent cliffs. It 

 is "usually cracked, and its color is that of a red, bluish gray lime- 

 stone, which is of a lighter color on the weathered surface. The 

 beds are often cut by vertical joints and there may be occasional 

 layers of shale. The Pentamerus limestone in Albany county has 

 an average thickness of 65 feet. It is a w^ell known formation 

 and has been quarried for lime at numerous points throughout the 

 state. One of the best exposures of this stone is at the Indian 

 Ladder. Underlying the Pentamerus bed is a series of thin 

 beddf^d. dark blue limestones, which generally crop out at the 

 base of the Helderberg escarpment but are frequently hidden by 

 the talus at the base of the cliff. These Tentaculite limestones 

 are often of a shaly nature. Their thickness along the eastern 

 face of Helderberg mountain according to Darton is about 30 

 feet. They are also exposed at the Indian Ladder and at South 

 Bethlehem. An excellent section of both Pentamerus and the 



