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NEW YORK STATE MUSEUM 



outflowing streams, leaving only the largest and most advan- 

 tageously situated of these in their old courses. 



It has been shown that the later Mesozoic was a time of wide- 

 spread base-leveling over much of the eastern United States, 

 notably in the Appalachian region and in southern New Eng- 

 land. While subsequent wear has removed much of that old 

 surface, the many fragments that yet remain indicate, by their 

 concordant summit levels and by their level ridge crests, that 

 they are remnants of a former plain; and that it was an erosion 

 plain is shown by the fact that its surface is notably discordant 

 with, or bevels, the rock beds. An extensive erosion plain of 

 the sort can only be produced, on a land surface, at stream 

 grade, and during a protracted period of comparative stability 

 of level. 



It has been further shown that this plain has been tipped 

 by subsequent movements, the evidence for which is the present 

 diminution of altitude in certain directions. Thus the uplift 

 of the Cretaceous peneplain of the Appalachians was greatest 

 along a n.n.e.-s.s.w. axis, from which the old surface drops 

 both to the east and to the west. The uplift was also unequal 

 along the axis, being greatest in Virginia and descending both 

 to the north and the south. The peneplain of southern New 

 England, which is supposedly of the same age, is strongly tipped 

 toward the south. 



This uplift was followed by another period of comparative 

 stability, during which large progress was made an reducing the 

 surface of these districts to the new base level. This interval 

 was not, however, so protracted as the previous, so that the 

 region was only partially base-leveled. Broad valleys were 

 opened on the weak rock belts, while but little progress was 

 made in the reduction of the resistant rocks, which remain sub- 

 stantially at their previous level, and form today the remnants 

 of the Cretaceous peneplain. 1 The widely opened valley bot- 

 toms on the weaker rock belts, with their concordant altitudes 

 when compared with one another, furnish the main evidence for 

 a long stability of the region at this grade. 



Campbell has recently urged the presence of a base level intermediate 

 to these two, from evidence obtained in northern Pennsylvania and southern 



New York. Geol. Soc. Am. Bui. 14 :27T-9G. 



