468 A. W. GRABAU THE SHERBURNE SANDSTONE 



the bar extended into the region of the estuary into which the Genesee 

 muds were washed^ and this probabh' closed the embayment on the south 

 and prevented the persistent Hamilton fauna from migrating around the 

 lower end of the bar and so reentering the central Kew York region. 



The eastern shore of this arm of the Atlantic from Green Pond Moun- 

 tain northward was the site of the great Catskill delta or alluvial deposit, 

 which had begun to form even before the Hamilton period had come to 

 ^n end, as will be more fully detailed later. 



When the Sherburne Bar became submerged, later on, the Hamilton 

 fauna which jDersistecl in the eastern area passed into the central basin, 

 there to commingle with the new indigenous fauna to constitute the 

 typical Ithaca fauna. This now was limited on the west by the deposits 

 which carry the pelagic Naples fauna, with which it interfingered, as 

 sliown by Clarke, and on the east by the encroaching continental fan of 

 the Oneonta. The Portage was brought to a close by the exclusion of the 

 Xaples fauna from the north and by a reinvasion of the lowan fauna 

 from the west and the Atlantic Hamilton fauna from the east. By a com- 

 bination of these faunas with the residual Ithaca fauna and renewed 

 invasion of foreign types, the typical Chemung fauna came into ex- 

 istence. 



The various provinces of this i^eriod have been given by Prof. J. M. 

 Clarke in a map showing their distribution, as far as New York State is 

 concerned (Memoir 6, Xew York State Museum, Plate B). This map 

 indicates the maximum extent of the Oneonta as far as the region of the 

 Sherburne Bar. Before this a part of the area colored for Oneonta was 

 occupied by the residual Hamilton fauna and later by the mixed Hamil- 

 ton-Ithaca fauna, as described in the preceding pages. Towards the end 

 the Oneonta beds covered this entire region. 



The Ashokan Formation or the so-called Sherburne Sandstone 



OF the Helderbergs 



Underlying the Oneonta formation of Ulster and Green counties is the 

 principal flagstone-bearing formation of the Hudson Kiver bluestone 

 region. This is the Lower Flagstone series of Darton and it has a thick- 

 ness of about 500 feet. In it are situated most of the important flag- 

 stone quarries of Ulster County, the most extensive and uniform deposits 

 of the bluestone occurring in the lower part of the series. Higher beds 

 often show characteristic cross-bedding of the torrential type, and irregu- 

 larity of bedding, thinning and thickening of layers, channelings, and 

 other current marks are common in certain portions. The series is unfos- 



