M.w \(.i;k STATE MUSEUM 



quite pure but containing lenses of chert, which 



chiefly abundant in the lower beds. At times it disappears 



altogether, but this La aol usual. < toe of the best ledges of Onon- 



ear Oniskethau creek at Olarks- 



vir 



].. ... ■•. p ]I> Id limestone. This reaches a large development 



in All 'any county and is divisible into several well marked mem- 



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



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

 and full of fossils. It- average thickness in Albany county is 

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



-iven of the same Led occurring at Rondout, Ulster co., and 

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



h of Callanans Corners. Underlying the Becraft limestone 

 Lb a series of different beds, of very impure, highly fossiliferous, 

 shaly lime-tone of a gray and -ravish 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 ITelderberg 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- 



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



re often cut by vertical joints and there may be occasional 



layers of shale. The Pentamerus limestone in Albany county has 



an average thickness of G5 feet. It is a well known formation 



and has been quarried for lime at numerous points throughout the 



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

 Ladder. Underlying the Pentamerus bed is a series of thin 

 bedded, 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 Bhaly nature Their thickness along the eastern 

 »f Belderberg mountain according to Darton is about 30 



feet They arc also exposed at the- Indian Ladder and at South 

 Bethlehem. An excellent section of both Pentamerus and the 



