of the Northern Catsh'dl Mountains. 



139 



escarpments toward the lowlands 

 on the north and east and gentle 

 dip slopes or platforms toward the 

 south and southwest. 



The physiography of the region 

 has been complicated somewhat 

 by at least two cycles of erosion. 

 During the first, the northern 

 part of the area was reduced to a 

 plain of advanced old age (a pene- 

 plain), while the southern part — 

 the Catskills, — being underlain by 

 more resistant rocks, was carried 

 only to the stage of late maturity 

 with a relief of 1500 to 2000 feet. 

 During the second cycle the 

 streams, intrenching themselves 

 to a depth of about 1000 feet, 

 have dissected the peneplain to 

 the stage of late youth, while in 

 the Catskills they have excavated 

 steep-sided valleys in the bottoms 

 of the broad, late mature valleys 

 of the previous cycle. This con- 

 dition, however, is not found in 

 the extreme upper courses of the 

 streams. Here the features char- 

 acteristic of the preceding cycle 

 still persist (fig. 2, right center), 

 either because trenching has not 

 yet progressed so far or because 

 t' .iew cycle has brought about 

 so o.xght a lowering of the profile 

 of equilibrium of these headwater 

 streams that widening has practi- 

 cally kept pace with downcutting. 



A subsequent glaciation does 

 not seem to have modified mate- 

 rially the broader features of the 

 physiography. 



The cuestas already mentioned 

 constitute the most conspicuous 

 elements of the topography. From 

 northeast to southwest each rises 

 higher than the preceding and 

 represents the outcrop of younger 

 rocks. The most northerly of 

 these, the Held erlj erg Cuesta, is 



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