856 



PRINCIPLES OF STRATIGRAPHY 



region of the present Wasatch Mountains. The basal Palaeozoic 

 sandstones of eastern North America, from a few feet to over a 

 thousand feet thick ("Potsdam" sandstone), also have many char- 

 acters pointing to such an origin. In this case, of course, the trans- 

 gressing Palaeozoic sea modified the deposits to a certain degree and 

 redeposited a part of them as fossiliferous marine sands and clays. 



"If a change from an arid toward a moister climate causes a 

 drainage discharge to the sea, a dissection of the plain will ensue. 

 The valleys thus eroded cannot expectably exhibit any great de- 

 gree of adjustment to the structures, because the stream courses will 

 result from the irregular patching together of the preexisting 

 irregularly disintegrated drainage. This peculiar characteristic, 

 taken together with the absence of neighboring uplifted marine de- 

 posits, will probably suffice in most cases to distinguish desert plains, 

 dissected by a change to a moister climate, from peneplains dissected 

 in consequence of uplift ; but there still might be confusion with 

 peneplains dissected by superposed streams." (12:401.) 



Locally, around individual mountains in an arid climate, a sur- 

 face sloping outward in all directions partly due to erosion and 

 partly to deposition is produced by the forces operative under such 

 conditions. Such a plane, though never very perfect, will have the 

 appearance of a broad-based cone — the center of which is the un- 

 dissected mountain remnant. Dr. Ogilvie (21) has described these 

 as forming around the laccoliths of the Ortiz Mountains in New 

 Mexico — and has named them "conoplains." They are essentially 

 elements in the stages of desert planation. 



C. MINOR EROSION FEATURES. 



Many of these have already been noted in previous chapters. 

 We may recall the grooves formed by eolian corrasion in the Libyan 

 limestone plateau and the erosion needles capped by Operculina 

 in the Libyan desert (p. 52) ; the Yardangs of central Asia and 



Fig. 225. Erosion features (Schichtenkopfe) in inclined Cretacic limestones. 

 Chiefly eolian. Abu Roasch. (After Walther.) 



