GEOLOGY AND PALEONTOLOGY OF THE SCHOHARIE VALLEY 223 



slopes of part of the hills, also the valley near North Blenheim, 

 where the characters of the two formations blend.^ 



The Ithaca formation 



The Sherburne flags are succeeded by a series of shales and 

 sandstones averaging 500 feet in thickness. These constitute the 

 Ithaca formation, the age of which in this section is middle Port- 

 age. It contains a modified Hamilton fauna, Avhich in some sec- 

 tions made its first appearance in the Sherburne. T r o p i d o - 

 1 e p t u s c a r i n a t u s is often abundant, ais is also S p i r i f e r 

 m u c r o n a t u s . Other characteristic fossils are S p i r i f e r 

 m e s a s t r i a 1 i s [fig. 180], characterized by radiating striae on 

 the rounded fold and sinus as well as on the flattened plications 

 of the rather large valves ; O r t h o n o t a u n d u 1 a t a [fig. 181] , 

 a pelecjpod shell of elongate form, with long and straight hinge 

 line parallel to the basal margin, and with strong undulations or 

 wrinkles in the posterior portion, which is delineated b}^ a pro- 

 nounced umbonal ridge ; S p h e n o t u s t r u n c a t u s [fig. 182] , 

 a ismall elongated pelecypod Avith angular umbonal ridge, trun- 

 cated posterior end, and fine surface striae, and S p h . c u n e - 

 at us [fig. 183], a larger species with more pointed anterior and 

 more rounded posterior end, and an additional pronounced ridge 

 above the umbonal one. With these are other but rarer species 

 as given in the list in chapter 7. 



This formation, like the Sherburne, passes Avestward into the 

 Naples beds, this change being effected be^^ond the meridian of 

 Cayuga lake. In this western region the Ithaca fauna extends 

 also much higher up, occupying most of the remaining Portage 

 beds. 



East of Schoharie county the typical Ithaca conditions dis- 

 appeared earlier, and along the eastern front of the Helderbergs 

 the Oneonta beds rest directly on the Sherburne if not on the 

 upper Hamilton. Thus the lower Oneonta beds of the east are 

 the equivalents in time of the Ithaca beds of the Schoharie region 

 and farther west, while the higher Oneonta beds replace the upper 



^Prosser, loc. cit, p. 205. See also map accompanying Prosser's article. 



