GEOLOGY OF THE NEW YORK CITY AQUEDUCT 



39 



The dividing lines between the different sandstones and shale 

 formations, the Oneonta, Ithaca, Sherburne, Hamilton and Mar- 

 cellus, can not be sharply drawn in the Esopus region. Together 

 they form in a large way a rather satisfactory field unit. For 

 specific purposes it is necessary to recognize that the lower po^r- 

 tions are prevailingly shales with thin bedded sandstones while the 

 upper portions are much more heavily bedded, the sandstones pre- 



Fig. 2 Spirifer mucronatus (Conrad), a characteristic and abundant index 

 fossil of the Hamilton shales of the Catskill margin 



vailing. The five divisions may possibly be more satisfactorily 

 made on paleontologic characters than on physical, but in most of 

 the advisory reports on economic and practical problems involving 

 this district the subdivisions can not be emphasized. The whole 

 series is essentially conformable and is very little disturbed [see 

 report on bluestone quarries, pt 2]. 



(10) Onondaga limestone. A bluish gray, massive, thick bedded 

 cherty, somewhat crystalline limestone. It is strongly marked off 

 from the Hamilton and Marcellus above, and, because of its greater 

 resistance to erosion, usually forms a dip slope controlling stream 

 adjustment and ultimately inducing the development of unsymmet- 

 rical valleys w4th gentle easterly slopes and clifflike westerly borders 

 where the streams are sapping the overlying Marcellus and Ham- 

 ilton shales. It is not sharply separable from the Esopus below but 

 everywhere in this region graduates into it with increase of silicious 



