101 J. Barrel! — Upper Devonian Delta of the 



naturally to an idea in regard to the competency of mountains 

 to endure through Mesozoic time which would be hard to adjust 

 with the conclusions advanced in this article regarding the large 

 degree of the destruction of relief in each of the earlier periods. 



The error in assuming one baselevel and a single uncompleted 

 erosion cycle for Jurassic, Comanche, and Cretaceous time 

 can be appreciated best by an examination of the Coastal plain 

 in Maryland and Virginia. _ At Washington the base of the 

 Potomac can be followed eastward under the Coastal Plain by 

 means of deep wells, that at Meadows penetrating 1511 feet 

 without entering the crystalline rocks below. Irregularities in 

 the margin and the elevation of outliers seem to show a relief 

 to the floor amounting to at least 300 feet, possibly 500 feet, so 

 that the slope as determined by wells is subject to this correction. 

 The larger number of wells and their extension over a zone 

 upwards of twelve miles wide unite in showing, however, that 

 the floor of the Potomac slopes southeastward at Washington 

 about 112 feet per mile. The deposits indicate a fluviatile 

 origin and this surface represents therefore the present inclina- 

 tion of the nearly horizontal baselevel at the opening of 

 Comanche time. The base of the Cretaceous is a marine plain, 

 sloping originally seaward at a grade of perhaps five or ten 

 feet per mile. It is now inclined 33 feet per mile. Clearly one 

 baselevel did not persist through the two periods, since these 

 two planes of deposition are inclined to each other at an angle 

 of 79 feet per mile. 



The base of the Potomac is laid upon the floor prepared by 

 Jurassic erosion. The volume of the Potomac gives some 

 indications of the volume of Comanche erosion; and the base 

 of the Cretaceous measures the warping which went forward 

 here during Comanche time. The Potomac group is given in 

 the Patuxent folio as from 510 to possibly 665 feet thick in 

 the region of its outcrop. In the deep well at Fortress Monroe, 

 65 miles east of the margin of the coastal plain, it is given in 

 the Norfolk folio as 1300 feet in thickness. As the Potomac 

 group consists of freshwater deposits, the sediment must have 

 been more than sufficient to fill the down warping on the 

 southeast* and implies a Comanche uplift to the northwest so 

 great as to separate completely the Jurassic and Cretaceous base- 

 levels. Most of the residuals above the Cretaceous peneplain 

 probably lie below the Jurassic baselevel of erosion and do not 

 imply therefore their continuous endurance above baselevel 

 since the Permian nor even since the close of Triassic time. 



The complete peneplanation of softer formations during 

 Cretaceous time and the large attacks made even on the 



*Barrell: Criteria for the Recognition of Ancient Delta Deposits, Bull. Geol. 

 Soc. Am., xxxiii, 405-411, 1912. 



