AGE OF THE BEEPS 345 



abundant small, nearly vertical pot-holes, and the sharp-edged rims in 

 the flat valley floor. 



AoE OF "Defps" 



Since the "deeps" have been cnt in tlie incipient Snsqnehanna pene- 

 plain an attempt has been made to correlate them, by means of the pene- 

 plains, with other incidents in geologic history. When Davis- described 

 the Somerville peiicphiiii and ('ampl)ell" the Harrisbnrg, the topographic 

 sheets covering the region between Ilarrisburg and the Chesapeake wei'e 

 lacking. The last of tliese, the McCalls Ferry sheet, was' published in 

 1912 and is now available. A study of these later sheets from Plarrisbiirg 

 to tide enables one to trace the previously recognized peneplains down- 

 stream and to correlate them more or less closely with the Coastal Plain 

 deposits of Maryland. The Harrisburg, which was regarded by C^ampbell 

 as early Tertiary, corresponds to the Lafayette or Brandy wine, now re- 

 garded as late Pliocene or early Pleistocene; the Somerville to the Sun- 

 derland; the Susquehanna in its two levels (one usually above aiul the 

 other below water level) to the Wicomico and Talbot, or, with equal 

 probability, to two phases of the Talbot. If this suggested correlation is 

 confirmed by later study, then the cutting of these "deeps" must be post- 

 Talbot in age and continue well into the Recent period. 



Conclusion 



The later history of the gorge may be sketched as follows : 

 Since Tertiary time the region has been part of the southeasterly 

 sloping Atlantic plain, largely reduced to a grade of approximately 5 

 feet to the mile. During the intervening period the land has been grad- 

 ually rising through a series of asymmetric oscillations until it is now 

 perhaps 500 feet above its former level. This elevation has been spas- 

 modic, witli periods of quiet when the sea, the master streams, and side 

 sti'eams have more or less completely reduced their confines to grade and 

 formed the well known Harrisburg, Somerville, and intermediate and 

 later imperfect peneplains. It was during or just subsequent to the latest 

 halt — thought to be more or less analogous to the Talbot terrace of Mary- 

 land- — that the Susquehanna, by increased volume or constricted channel, 

 developed six or more "narrows" in which the erosive action of the river 



- W. M. Davis: Boston Soc. Xnt. Ilisr., TMoc, vol. 24. 1880, p. 392. 

 " M. R. Campbell: Bull. Geol. Soc. Am., vol. 14, 1903, pp. 277-296. 



