750 N. M. FEN NEMAN 



aggraded it recently, but locally the same phenomena are well exempli- 

 fied where the valley is distinctly terraced, the terraces and flood-plain 

 all sloping toward the stream indicating progressive down-cutting. 



If rills running over loose materials may be regarded as overloaded 

 streamlets, and if it be assumed that the surface of a ridge is washed 

 by such streamlets behaving as here described, and capable of carry- 

 ing away from a given point more material than they bring, we have 

 the conditions for the continued down-cutting of broad slopes without 

 cutting valleys and for the lowering of a ridge without its subdivision 

 into hills; it may be at a rate which is uniform throughout its length. 

 Thus a broad area consisting of hills and ridges of uniform height 

 may be cut down in such a manner as approximately to preserve their 

 uniformity of height and the flatness of the sky line. 



St. Louis peneplain. — Before going further it may be stated that 

 this discussion was not begun with an academic interest, but in an 

 attempt to explain what appears to be a case of just such uniform 

 down -cutting as is here assumed. The area in question is the St. 

 Louis quadrangle and adjacent territory. It is a low plateau with a 

 mature drainage system. The ridges and very narrow remnants- 

 of upland rise to such a uniform height that no physiographer would 

 hesitate to pronounce them the remnants of a former plain of very 

 faint relief (in this case a peneplain as shown by abundant evidence). 

 Furthermore the supposition would be that the horizon of the former 

 plain was approximately that of the present hifltops. 



Above the uniform level of the hilltops are a few exceptional 

 elevations of fifty feet or more. Capping these are deposits of typical 

 Lafayette gravels from ten to twenty feet thick. No theory of the 

 origin of the Lafayette formation which receives any credence, ad- 

 mits the supposition that these gravels might have been deposited 

 on exceptionally high points in preference to a lower surrounding 

 plain. If the elevations on which they now rest existed as such when 

 the gravels were deposited, it would seem necessary to assume that 

 the entire surrounding plain was buried by gravel to a depth equal 

 to the height of these hills plus the thickness of the deposit on the 

 hills. It must then be assumed that subsequent erosion was guided 

 in such a manner as to strip practically the whole of this thick bed 

 of gravel from the surrounding plain while leaving the thin deposit on 



