CYCLES OF EROSION IN PENNSVLVANIA 543 
periods in the Appalachian highlands has been recognized by 
Keith,* Campbell,? and others. 
In the sedimentary sequence of the Atlantic plain there are ten 
significant unconformities—that is, ten intervals of erosion alternat- 
ing with intervals of deposition. Not all of the deposits are known 
to be marine, so that ten submergences cannot be postulated. 
There are six less significant unconformities. 
The time represented by the deposits and unconformities has 
been estimated at 56,500,000 years. There could not conceivably 
be conditions more favorable for a succession of erosion cycles 
falling so far short of completion as to leave permanent traces of 
the sequence, nor a stratigraphic record more compelling for the 
acceptance of such traces as evidences of erosion cycles. It is 
not probable that traces are preserved of every incomplete erosion 
cycle. 
The topography of erosion cycles early interrupted would be 
obliterated by subsequent erosion cycles of longer duration. Small 
beginnings, if they existed, might be quite similar to the three most 
recent terraces, which are being modified and will in time be 
completely obliterated by subsequent erosion. 
Such incomplete, obliterated cycles may be registered only in 
the lesser unconformities of the stratigraphic record, which is 
easily more complete than the topographic record. Did the 
geologist base his expectations on stratigraphy alone, he would 
look for a series of more or less discontinuous and more or less 
warped benches or terraces facing the sea, or, in the case of the 
lower terraces, following inland the river valleys, and not perfectly 
stairlike because each terrace will have its peculiar angle of slope. 
The terraces are the topographic record of the succession of inter- 
rupted erosion cycles of which the unconformities in the strati- 
graphic sequence are the geologic record. 
t Arthur Keith, “Some Stages of Appalachian Erosion,” Bulletin Geol. Soc. 
America, Vol. VII (1806), pp. 519-24. “‘Geology of the Catoctin Belt,” Fourteenth 
Annual Report, U.S. Geol. Sur., Part II (1892-93), pp. 285-395. 
2M. R. Campbell, “‘Geographic Development of Northern Pennsylvania and 
Southern New York,” Bulletin Geol. Soc. America, Vol. XIV (1903), pp. 277-96. 
3 Joseph Barrell, ““Rhythms and the Measurement of Geologic Time,” Bulletin 
Geol. Soc. America, Vol. XXVIII (1917), pp. 745-904. ' 
