582 E. W. SHAW AGES OF APPALACHIAN PENEPLAINS 



tildes, and that, sloping seaward, it passes beneatli deposits of a certain 

 age, but it is rare that the slope of the Imried surface for even a short 

 distance is set forth. 



However, from published and unpublished observations together, the 

 general lay of the buried peneplains can be ascertained fairly satisfac- 

 torily. One in particular — the floor uixder the Cretaceous formations — 

 can be determined with considerable accuracy. The facts come from (1) 

 outcrops in a belt along the landward side of the Coastal Plain, (2) well 

 records in this belt, and (3) well records in the middle and seaward por- 

 tions. They show that to the east, south, and west this surface slopes 

 away from the Appalachians at a rate generally about 30 feet to the mile. 

 In places the slope is steeper. In the District of Columbia it is fully 100 

 feet to the mile. In a ie^Y places, as in North Carolina, where there has 

 been U23lift near the j^resent coast, it is more gentle. But such departures 

 are not enormous and are found in only a few places. The slope as a 

 whole seems remarkably uniform. 



On the other hand, according to all descriptions and maps, the stopes 

 of all peneplains about the adjoining margin of the Appalacliian province 

 is much less, ranging from 5 to 15 feet to the mile. This is especially 

 noteworthy and relial)le in the south, where there has been little deforma- 

 tion. Therefore, testimony l)ased on projected planes indicates that the 

 floor under the Cretaceous deposits is older than the so-called Cretaceous 

 13eneplain ; older even than the monadnock tops supposed to rise above the 

 surface. Even the unconformity at the top of the Cretaceous seems to 

 have a steeper slope than the adjoining portion of the so-called Cretaceous 

 peneplain. 



If it be argued that a gradual increase in slope toward the sea is to be 

 expected, it may be replied that according to descriptions and general 

 altitude data the slope of exposed peneplains in a broad belt adjoining 

 the Coastal Plain decreases seaward, and the belt of greatest slope is many 

 miles landward from the Coastal Plain border. Probably all will agree 

 that cross and longitudinal profiles of Appalachian peneplains would 

 show, as a general rule, a nearly horizontal central portion from whicli 

 the slope increases in all directions outward for some scores of miles, be- 

 yond which the inclination decreases as the altitude approaches sealevel, 

 the central mountainous portion having suffered much the greatest uplift. 

 Indeed, there is a generally accepted inference that in the upper portion 

 of the Mississippi basin two or more of the peneplains, each approaching 

 sealevel (or local baselevels not far above sealevel) at a declining rate, 

 become so nearly coincident that they can not be distinguished. 



Downwarping due to isostatic adjustments can scarcely save the good 



