5G4 W. M. DAVIS^DATES OF TOPOGRAPHIC FOEMS. 



the Atlantic slope, namely, to the elevation that the peneplain suffered 

 after its production by the forces of subaerial denudation. 



The Tertiary Elevation of the Cretaceous Peneplain. — The absence of Juras- 

 sic deposits on the Atlantic slope has already been alluded to as evidence 

 that during that period the shore-line stood to the seaward of the present 

 inner margin of the Cretaceous formation. How much further it lay to the 

 seaward cannot be told. It has also been explained that when the Creta- 

 ceous submergence occurred, the coastal lands had been reduced to moderate 

 elevation and faint relief by long-continued denudation, and that for this 

 reason a slight submergence caused a broad inland transgression of the sea. 

 The farthest inland reach of the Cretaceous shore has yet to be carefully 

 worked out. In southern New England there are no Cretaceous deposits 

 on the mainland, but on Long island and the other islands farther eastward 

 the edges of this formation are found. Its inland extension for many miles 

 is therefore eminently possible, and must be measured by the degree of dis- 

 cordance with the structure that the streams of the coastal slope exhibit. 

 Connecticut offers a promising field for this study. In New Jersey my own 

 studies have given something of definiteness to this aspect of the problem ; 

 the rivers there do not show any trace of discordance with the structure of 

 their basins except east of a narrow marginal belt of the highlands. In 

 southeastern Pennsylvania McGee has shown that the eastern division of 

 the crystalline belt was in good part buried under the Cretaceous formation ; 

 the transverse gorges of the Neshaminy, Wissahickon, Brandywine, and 

 other relatively small streams may be taken to indicate superimposition,* 

 but the inland margin of the area of discordant streams is not yet well de- 

 fined. In the neighborhood of Philadelphia the superimposed quality of the 

 smaller streams is well marked, and with the appearance of good maps 

 typical examples of this interesting relation can be taken from that district. 

 Farther southward no discussion of this question has yet been published. 



The changes of level up to the end of the Cretaceous period may be gen- 

 eralized in figure 5. Here w^e have, in rough diagram form, the land and 

 water profile resulting from the Jurassic deformation of the Atlantic slope 

 (1, 1, 1), the denudation of this form (2, 2) and the advance of the sea in 

 early Cretaceous time, and the late Cretaceous profile of the low peneplain 

 (8), with its overlapping sediments. This brief summary neglects the Poto- 

 mac oscillation, detected by McGee, as being too early to find expression in 

 existing topographic features. 



The subsequent greater changes of level, chiefly in the sense of elevation, 

 are more directly connected with our problem. They are generalized in 

 figure 6, whose uppermost profile corresponds to the lowest profile of figure 5. 

 The results of the elevation of the Cretaceous peneplain are seen, first in the 



*See Germantown, Pa., and Burlington, N. J., atlas sheets, United States Geological Survey. 



