558 F. BASCOM 
distribution: the areal succession of peneplains from interior to 
coast would exactly accord with the chronological succession. 
The region is, however, one of varied structural and lithologic 
resistance to weathering and the peneplains are not therefore so 
simply spaced; younger peneplains on relatively weak rocks are 
found inland at higher altitudes than the marginal remnants of 
older peneplains. This fact would be still more apparent if the 
extreme margins of the older peneplains, now buried beneath 
sedimentary formations, were shown. 
The question of the origin of these neneetiie that is, of the 
nature of the dominant erosive agent, is open to debate. The 
Cross Section 
1000 Brandywine River 
750° 
MERE Pa ‘ 
500 Bee ry peace randywine 
250° 
0 
Fic. 23.—Section crossing Brandywine River 
three youngest terraces, in Maryland presumably-of marine origin, 
are in this region of fluvial-estuarine or of fluvial origin; that is, 
they were developed on the borders of the Delaware estuary or on 
a shrinking meander belt of Delaware River. 
That the Late Brandywine is of subaerial origin is concluded 
from the evidence of the valleys, now submerged, which extend 
across the continental shelf and which it is believed were excavated 
in Late Brandywine time. 
That the five peneplains are in part of marine and in part of 
subaerial origin seems a warranted conclusion. Each peneplain 
was partly submerged and carries marine sediments, but there does 
not seem to be sufficient proof that any one peneplain was com- 
pletely submerged. They parallel the coast line as would be the 
case were they of marine origin, but this may also be true of sub- 
aerial peneplains, and the great inland extension of the Kittatinny 
