74 C. R. KEYES INTERMONT PLAINS OF THE ARID REGION 



The other misleading point is that pebbles do actually appear to occur 

 everywhere over the plains. This aspect is almost always very deceptive. 

 Gravel and small boulders are to be found scattered over the higher slopes 

 of nearly all of the intermont valley plains. That they give to the loams 

 the appearance of gravel beds is due to the fact that the winds constantly 

 blow away the fine materials. The pebbles left behind serve as a protec- 

 tion to further wind action until a floodsheet or torrent disturbs the 

 coarse surface layer of small stones. It is not uncommon to find areas 

 several acres in extent covered with small angular stones as closely and as 

 evenly set as mosaics. The pebbles may be only one deep; below is fine 

 porous loam, into which man and beast in crossing sinks as into newly 

 fallen snow. Turned up by the plow, the most barren tract of this kind 

 may prove to be the finest of loamy soil. 



The almost gravelless plains detritus and the thin mantle over a 

 beveled rock-fioor observed in the arid regions make one's notions derived 

 from experiences in humid lands imdergo radical modification. 



SiGNIFICAjSTCE of a THINLY MANTLED EOOK-FLOOR 



With the bedrock of the broad intermont plains possessed of the same 

 geologic structures as the mountains, only thinly covered instead of 

 deeply buried by detritus, and formed on a surface evenly planed ofi', the 

 main interest centers around the agencies and conditions under which the 

 planation was accomplished. 



In the American region, with its fine patterned relief and the general 

 plains surface greatly disturbed by recent faulting on a large scale, the 

 even rock-floor worn out on the beveled edges of the rock strata long 

 escaped merited attention. In southern Africa, where there is the same 

 elevated plateau and the same dry climate, but where the region has not 

 undergone notable deformation and dislocation for a long time, Passarge^" 

 has made the deduction that general planation may go on over a vast area 

 without peneplanation ; and that the leveling may be more perfect than 

 is possible in the case of normal baseleveling. In the light of this sug- 

 gestion, the existence under thin detrital covering of even rock-floors in 

 the plains of the much disturbed region of the Great basin and the Mexi- 

 can tableland acquires a new meaning. 



Origin of the Eock-floor Surface 



penepla-natwn 



At the time at which the beveled character of the rock-floors of some 

 of the New Mexican bolsons was first recognized, in 1903, these even 



" Zeitschrift der deutsche geologische GeseUschaft, Ivi band, Protokol, 1904, p. 196. 



