144 J. L. Rich — Physiography and Glacial Geology 



Jurassic, -erosion planes.* It seems more likely, since it 

 corresponds very closely with the stratilication, that it is mainly 

 structural. Furthermore, such an even skyline is to be 

 expected, irrespective of former peneplanation, in the late 

 mature development of a region of homogeneous rocks, f 



Certain features of the 2000 foot plateau which borders the 

 Catskills on the north lend support to the view that it is a 

 structural feature rather than an erosional plain, but other 

 features pointing toward the latter hypothesis seem more 

 conclusive. Of the features which might be interpreted as 

 indicating a structural origin the following may be mentioned: — 



(1) The plateau either extends flat or, locally, rises gradually 

 toward the north from the base of the Catskills to the crest of 

 the Hamilton Escarpment, then drops down 400 to 500 feet to 

 another platform lying at elevations between 1200 and 1500 feet 

 and developed on the Devonian limestones. This in turn breaks 

 down into a local erosion level at about 700-800 feet which 

 borders the youthful trench of the Mohawk River. The close 

 accord of at least the higher of these steps with the outcrops 

 of resistant members of the rock series leads one to believe 

 that structural control has at least been strong. 



(2) In several places, notably near G-ilboa and Grand Gorge, 

 the flat-topped hills marking the 1800-2000 foot level are 

 capped by resistant beds of sandstone which, it might be urged, 

 are entirely sufficient to account for the flat-topped plateau in 

 that vicinity. 



Opposed to the structural hypothesis is the fact that accord- 

 ing to the profiles on the Geological Map of New York State, 

 1901, the 2000 foot upland bevels the gently dipping beds, 

 and consequently cannot be determined throughout by resistant 

 strata, however pronounced this feature may be locally. 

 Furthermore, certain features of the valleys in the plateau and 

 in the Catskills to the south indicate rejuvenation at a relatively 

 late period and therefore lend support to the peneplain 

 hypothesis. In the Catskills, for instance, the valleys of Town 

 Brook (Hobart sheet, U. S. G. S.) and Batavia Kill (Gilboa 

 and Durham sheets) are broad and open (i.e. late mature) in their 

 upper parts within the mountains, while farther down stream, 

 below the 2000 foot level, they take on the characteristics of 

 youth : the sides become steep, and the valleys narrower. 

 Batavia Kill may be taken as an example. Above Windham 

 (1500 feet) the valley has every characteristic of late maturity : 

 there is considerable relief, but the valley is broad and open, 

 with steepest slopes near the divides. Followed down stream, 

 despite the fact that it is leaving the highest mountains, the 



* Compare Barrell, this Journal (4), xxxvii, p. 104. 



f Shaler, N. S., Bull. Geol. Soc. Amer., x, 263-276, 1899. 



