NOTES ON THE ORIGIN OF CERTAIN PALEOZOIC 
SEDIMENTS, ILLUSTRATED BY THE CAMBRIAN 
AND ORDOVICIAN ROCKS OF CENTER COUNTY, 
PENNSYLVANIA 
THOMAS C. BROWN 
Bryn Mawr College 
During the Cambrian period the North American continent 
subsided relative to sea-level. Before the close of the period epi- 
continental seas had transgressed the greater part of the continent 
now included within the borders of the United States, and, through 
the Upper Cambrian at least, limestone-forming conditions pre- 
vailed somewhat extensively in the central and southern Appala- 
chian region. Whether this apparent subsidence was due to 
diastrophic movements or to the more or less complete base-leveling 
of Cambrian continental areas is not altogether certain. The 
type of sediments accumulated during Lower and Middle Ordovi- 
cian time suggests that it was due to base-leveling rather than to 
diastrophism. In this central and southern Appalachian region 
great thicknesses of limestones accumulated between the Upper 
Cambrian>and Trenton beds. Clastic sediments are sparsely 
represented and those which are present are peculiar in character. 
Yet land areas could not have been very far distant to the east and 
northeast, because in adjacent states this very time interval is 
represented by an unconformity and period of erosion, as for 
example in New Jersey.‘ The most reasonable explanation then 
seems to be that the adjacent land area had been reduced during 
the Cambrian to a peneplain. As a result, the streams and rivers 
did not have sufficient velocity to transport any considerable 
amount of clastic material into the adjacent epicontinental sea. 
Weathering processes, however, were at work and a thick mantle 
of residual rock waste was being prepared all over the peneplained 
land surface, only waiting for renewed stream activity to be 
t Weller, Geol. Surv. N.J., Report on Pal., III (1903), 15. 
232 
