THE GEOLOGICAL HISTORY OF NEW YORK STATE 65 
ing, of the coastal lands to allow deposition of sediments over 
much of the region now known as the Atlantic Coastal plain. That 
but little downwarping of the surface was necessary in order to 
produce proper conditions for sedimentation is evident because the 
coastal lands, just prior to the Cretacic, were already low-lying as a 
result of the long Jurassic erosion period. There was just enough 
warping of these low coastal lands to produce wide flats, flood 
plains, shallow lakes, and marshes back from the real coast line and 
in which were deposited the sediments derived from the Piedmont 
plateau and Appalachian areas. The early Cretacic deposits thus 
formed are known as the Potomac series, and consist of very 
irregular layers of sand, gravel and clay. The very irregular ar- 
rangement of these beds and their rich content of fossil land plants, 
afford conclusive evidence that the sediments were not accumulated 
under marine conditions. The Potomac series outcrops at the 
western margin of the present Coastal plain and has been traced 
from Martha’s Vineyard, through Nantucket, Long Island, Staten 
island, Northern New Jersey, and southward into Georgia. Passing 
seaward the strata dip under those of later age (see figure 22). 
On Long Island, Potomac outcrops occur only along the north- 
western border but these beds no doubt dip under the more recent 
deposits of the rest of the island. The maximum thickness of the 
Potomac series is only about 700 feet. 
Along the Atlantic coast certain deposits which should come 
between the Lower and Upper Cretacic are missing, and the Upper 
Cretacic beds rest upon the eroded surface of the otherwise undis- 
turbed Lower Cretacic. Thus we know that there was a gentle 
upward oscillation of the land toward the end of the Lower, or 
beginning of the Upper, Cretacic, after which a moderate amount 
of erosion of the Lower Cretacie beds took place. 
Then came another gentle submergence of the coastal lands when 
the Upper Cretacic strata were formed. The character and present 
extent of these deposits, and the fact that they are of marine origin, 
prove that this subsidence allowed a shallow sea to spread over 
practically all of what is now called the Atlantic Coastal plain in- 
cluding most of Long and Staten islands in New York. Accordingly 
we learn that, for the first time since the close of the Paleozoic, 
did truly marine conditions prevail over any portion of New York 
State, and also that Appalachia, the great land mass of the east, 
wiich had persisted through the many million years of the Paleozoic 
and most of the Mesozoic, now disappeared under the Cretacic sea. 
