Relation of Streams to Bryn Mawr Gravel. — Ba.s-cow. 51 
of the changes which produced the plain upon which they are 
at work. The pre-Newark history of tlie plateau must be 
read from the deposits and the larger rivers. It quite precedes 
the birth and growth of the present streams. 
Briefly, the deposits of that portion of the Piedmont plateau 
traversed by the streams in question (north and west of Phil- 
adelphia in Montgomery, Chester, Delaware, and Philadelphia 
counties) indicate that it must have passed through some 
such sequence of events as is outlined below: In later Algon- 
kian and Lower Cambrian time it was a land surface. In 
Upper Cambrian time it was depressed and received a thin 
deposit of Cambrian and Ordovician sediments. 
Early in the Ordovician period it was elevated, forming 
part of a lofty mountain range, which furnished continuous 
supply of material during all remaining Paheozoic time to the 
western sea. Then followed, upon the western portion, at 
least, of what was now a peneplain, the Newark depression 
and later elevation,, reversing the direction of the rivers. Sub- 
sequent to this elevation occurred extended erosion, when the 
region was again reduced nearly to base-level. Thus, at the 
dawn of the Cretaceous, the Piedmont plateau was a plain 
slightly inclined seaward. From this point the story has been 
carried forward to mid-Cretaceous time by McGee.* Follow- 
ing upon the baseleveling mentioned above occurred, at the 
opening of Cretaceous time, the submergence which initiated 
the Potomac deposition. This deposit was at first a gravel, 
later with decreased declivity^ of the land the materials be- 
came finer. About the middle of Potomac time, there was, 
Mr. McGee thinks, in this region a temporary emergence of 
the land followed by submergence, when the upper member of 
the Potomac formation was deposited. 
Since the deposition of these clays and gravels and other 
possible members of the Cretaceous series the plateau has 
perhaps been permanently above water. 
What McGee has called the upper member of the Potomac 
formation is represented near Philadelphia by the plastic clays, 
which have been referred by the Second Geological Survey of 
Pennsylvania to the Wealden.f Some of the best exposures 
*McGee, W J: Three Formations of the Middle Atlantic Slope: Amer. 
Jour. Sci., volume xxv, 1888, pp. 142, 14.3. 
fReport X. Hand Atlas of Pennsylvania. J. P. Lesley, 1885, pi. 40. 
