834 PRINCIPLES OF STRATIGRAPHY 



glance at the geologic map of New York State shows a series of 

 color bands representing the various strata of an ancient (Palaeo- 

 zoic) coastal plain, the lowest appearing around the Adirondack old 

 land and along the border of the crystallines north of Lake On- 

 tario, while each later one is further and further removed, south- 

 ward, until the latest, the Carbonic strata, scarcely extend into New 

 York State at all. Now, most if not all of these strata once ex- 

 tended far toward, if not entirely to, the Adirondacks, and to the 

 Laurentian old land in Canada. Their present distant outcrop is 

 in large measure due to peneplanation across gently inclined strata. 

 Subsequent erosion has, of course, pushed the edges of many of 

 these strata further south than where they were left at the end of 

 the period of peneplanation, but the amount of this later removal 

 was small as compared with the greater separation of outcrops 

 effected by peneplanation. It was formerly thought that the out- 

 crop of the edges of strata along the margin of the old crystalline 

 land marked their former extent. Since the strata of eastern North 

 America crop out in a series of belts margining the old-land, each 

 later formation falling short of the preceding one, it was believed 

 that North America rose by a series of steps, the sea, at the end of 

 each period, retreating to the region near which the next later for- 

 mation now comes to an end. From the characters of the forma- 

 tions, however, it appears that they accumulated in a subsiding sea, 

 and that each formation in turn overlapped the preceding ones, with 

 few exceptions. 



The present appearance of the outcrops is due wholly to erosion, 

 the higher formations having suffered most. J\Iany of the later 

 Palaeozoic strata of the eastern United States derived their clastic 

 material from the Appalachian region on the southeast, and their 

 northwestward limit in some cases was far beyond the border lines 

 of the present Canadian old land, a great portion of which may in 

 fact have been entirely, submerged during a part of the Palaeozoic. 

 That the erosion of the strata continued until peneplain conditions 

 were reached is shown, not only by the fact that the remnants of 

 this old surface in the eastern L'nited States form parts of a some- 

 what warped plain rising soutliward, but also and more especially 

 by the fact that this surface is not formed by a single hard stratum, 

 but by various hard beds which have been beveled across. Thus the 

 Alleghany plateau of western New York, which is a characteristic 

 part of this old peneplain, is composed of the beveled edges of suc- 

 cessively higher southwestward dipping strata as shown in the 

 following diagram. (Fig. 207.) 



This beveling of the strata can be interpreted only as the result 



