Baseleveling nml Organic Evolution. — Woodworth 'I'l'.i 
steeper than that of the overlying marine Cretaceous. This 
fact supports the conclusion arising out of the absence of 
other than a few brackish water molluscan fossils in the group, 
that a barrier existed at this time eastward of the present 
shore-line, whereby the sea was partly excluded from the 
Piedmont area. This would make the Raritan and the older 
Potomac groups probably of lacustrine or estuarine origin de- 
posited in basins formed by the warping of the baseleveled 
plain. 
Post-Cretaceous history of the peneplain. 
At the close of the Cretaceous, the peneplain and the upper 
Cretaceous deposits which covered its submerged seaward 
margin were elevated and tilted. Streams began to cut clown 
valleys and to undergo those adjustments which had previ- 
ously been outlined in the initial stages of baseleveling. Great 
changes in geography were made in the western or cordilleran 
portion of the continent. By the end of Eocene times in 
eastern North America, the streams had almost worked out 
another or Tertiary baselevel ; renewed uplift caused them to 
sink their valleys still deeper. 
The coast shelf the correlative of Hn j peneplain. 
Denudation has its counterpart in deposition. The growing 
coastal shelf of the Atlantic coast, including the formations 
newer than the Jura-Trias and older than the Tertiary, is the 
structural complement of the adjacent portion of the merely 
superficial peneplain. If the one influenced the character of 
the land life during its topographical development, the other 
had its due effect on the marine fauna. Hayes and Campbell* 
have recently called attention to the correlation between the 
sediments and cycles of baseleveling in the southern Appala- 
chians. With the decline in the grade of the rivers debouch- 
ing on the shore, the sediments became finer and finer; when 
baselevel was reached the stream contribution was in the form 
of chemical rather than mechanical waste. With the progressive 
shallowing of the continental submerged margin the scour of 
currents and etfiux of tides would spread farther out from the 
shore-line the conditions which on deep water shores are con- 
fined to a narrow fringe below the littoral. The shoaling of 
*Gteomorphology of the southern Appalachians, Nat. « i»-< >ir. Mag., vol. 
vi. 1894, pp. 123-136. 
