UNCONFORMITY AT BASE OF ONONDAGA LIMESTONE 303 
and Schoharie formations which, in the Hudson and Delaware 
valleys, separate the Oriskany from the Onondaga above. In 
the east-central New York region the Onondaga limestone rests 
upon an old eroded land surface composed sometimes of the 
Oriskany sandstone, but much more frequently of limestone of 
the Helderberg group. The basal beds of the Onondaga if followed 
westward across New York are seen to rest successively on con- 
formable Schoharie in southeastern New York, disconformable 
Oriskany sandstone and limestone of the Helderberg group and 
finally upon Silurian strata in the western part of the state. The 
physical evidence of the disconformity in this region includes 
both an irregular or angular contact surface between the Onondaga 
and subjacent beds and a basal sandstone or conglomerate. The 
latter is usually less than a foot thick and frequently comprises 
only a few inches of calcareous sandstone with occasional fragments 
of limestone from the Helderberg below. This sandy bed at the 
base of the Onondaga frequently grades upward into the limestone 
and gradually merges itself into it. The maps and reports which 
deal with the Devonian in central New York usually refer this 
basal sandy bed beneath the Onondaga limestone to the Oriskany. 
There can, however, be but little doubt that it represents reworked 
Oriskany sand. But it cannot properly be referred to the Oriskany 
because Oriskany fossils are absent and Onondaga fossils are 
frequently present in it. This thin basal sandstone band at the 
base of the Onondaga is well exposed at the Splitrock quarry 
southwest of Syracuse. At the east end of the Splitrock quarry 
the 4 or 5 ft. of Onondaga limestone is separated from the Helder- 
berg below by a thin band of sandstone. The lower 2 in. of the 
sandy beds is probably 75 per cent sand in a calcareous matrix. 
The percentage of sand decreases and the lime increases upward 
gradually until all of the sand disappears within 1 ft. of the top 
of the Helderberg. Only ‘about 6 in. of the sandy band at the 
base of the Onondaga could properly be called a sandstone and no 
sharp line of demarkation between this and the slightly less sandy 
base of the limestone could be drawn. The absence of Oriskany 
fossils from this basal sandstone and the presence in it of Onondaga 
corals at the very base of the bed clearly indicate that it belongs 
