_ CYCLES OF EROSION IN PENNSYLVANIA 541 
In any investigation of the cycles of erosion, complete or incom- 
plete, that collectively constitute the erosion history of the Pied- 
mont province of the Appalachian highlands, the stratigraphic 
record preserved on the margin of the province must furnish the 
data by which the succession and age of such erosion cycles stand 
or fall. 
That aerial and marine erosion has taken place ever since 
continental plateaus and oceanic basins came into existence is 
unquestioned: only such interaction of air, water, and land masses 
is conceivable. ‘The character and rapidity of erosion will be con- 
trolled by altitude, rock, and climate, but the duration of an 
erosion period will be dependent upon the stability of the strand 
line: a long period of quiescence will permit prolonged aerial and 
marine erosion with reference to a given base-level, and a period 
of uplift will interrupt and renew erosion with reference to a new 
- base-level. The evidence of such movements of the strand line 
inaugurating erosion is furnished by the stratigraphic register. 
In the Piedmont province of Pennsylvania with the beginning 
of Cretaceous sedimentation the stratigraphic record seems to 
indicate a succession of such erosional conditions maintained by 
an alternation of periods of continental quiescence with periods of 
movement. That these periods of stability have been of different 
durations is an obvious deduction from the sedimentary record. 
The stratigraphic record on the Atlantic plain which is the 
submerged margin of the Piedmont province is as follows: 
. Recent deposits 
. Unconformity 
Pleistocene deposits 
Aialinota (Cape lVilay)ianclayaisand sand\eravelia meee ya aera 30 feet 
Unconformity 
Wicomico (Pensauken): clay, sand, and gravel................ 25 feet 
Unconformity 
Sunderland (Bridgeton): clay, sand, and gravel............... 25 feet 
Unconformity 
BelatesP randy wine: sand andygravellenit. yates esl iyore olerore 1+ foot 
Pleistocene (or Late Tertiary ?) deposits 
Unconformity ? 
Barly randywane-sandranducrayelanm mir ret termine ye 50 feet 
