BEVELING OF STRATA 



835 



of peneplanation, in other words, when the level of erosion was 

 reached, erosion could go no further because this surface stood so 

 close to sea-level that the streams could not cut lower. Subsequent 

 elevation w^ould permit of the cutting of lowlands on the softer 

 strata (5, 9 and 12), leaving the harder ones in relief. 



That this beveling of the strata is due to erosion and not to a 



r - - -LOntari 



n 



Fig. 207 



JO 9 8 7 6 5 4 



Section across New York State, from Ontario to the Pennsyl- 

 vania lino, showing the peneplanation of the strata along the 

 dotted line, and the subseqnent carving of valleys on the softer 

 strata, i, Archaean; 2, Potsdam or Beekmantown ; 3 and 4, Black 

 River-Trenton; 5, Utica-Lorraine ; 6, Qneenston ; 7, Medina- 

 Clinton-Rochester ; 8, Lockport-Guelph ; 9, Salina ; 10, Bertie- 

 Cobleskill; 11, Onondaga; 12, Marcellus; 13, Hamilton; 14, 

 Genesee-Portage-Chemnng. 2-6, Ordovicic; 7-10, Siluric; 11-14, 

 Devonic. 



shoreward thinning is show^n by the fact that the material of the 

 strata does not change toward the beveled portion as it would if that 

 part had marked a progressively retreating shore accumulation, 

 while the fact that the lowest portion of the beds extends farther 

 than the higher portions shows that the thinning cannot represent 

 an overlapping transgressive series. The following diagrams illus- 

 trate the thinning of the strata by overlap and by beveling. (Figs. 

 208, A B.) 



Fig. 208. Diagram illustrating the thinning of strata: A, by overlap; B, by 

 beveling through erosion. 



Ancient Coastal Plains Shozving Cucsta Topography. 



Looking over the geological maps of the world, many examples 

 of ancient coastal plain strata, in which a cuesta topography has 

 been revived after peneplanation, are found. The Taheozoic strata 

 of New York and Canada furnish excellent examples, though part 

 of the topography is drowned, or obliterated by subsequent de- 

 posits. (Figs. 209, 210.) 



