248 N. s. siFALKK — i,()i<:ss i)i':i'(»si'i's oh' Montana 



tnicG of wiiul-blowii detritus. The evidence, in a word, goes to show 

 that the passage from conditions of considerably greater rainfall than 

 exists at present to the existing arid state was (juickly and recently 

 brouglit about. This former rainfall gave the torrents much greater 

 transi)ortative force than they have at present, and thus maintained in 

 the region they occuiiied a .coating of vegetation sufficiently dense to 

 prevent the scouring action of the winds. As yet I have not succeeded in 

 finding any satis factor}' basis on which to found an estimate of the time 

 which has elapsed since this change occurred, but the impression made 

 upon the observer is that it cannot well exceed a few thousand years. It 

 may possibly have been coincident with the closing stages of the Glacial 

 period, but it appears to have been veVy much more recent. 



In certain parts of central Montana which I have carefully observed, 

 the record of the detrital cones can be traced back from the present day 

 to a time which clearly long antedates the last glacial i)eriod. This time 

 is recorded by cemented arkoses, so hard that the}' have been mistaken 

 by fairly good observers for the ancient schists, and associated thick de- 

 posits of volcanic ash, the latter particularly abounding in the valley of 

 Ruby river — ash derived from cones which are now reduced to ruin. At 

 no point in the many sections which I have examined are ancient de- 

 posits of loess shown. So far as this negative evidence may be trusted, 

 it indicates that the present arid condition of this part of the Cordilleras — 

 that is, the valley of the upper Missouri — is exceptional. 



The evidence that the time during which the existing dry condition 

 of central Montana has not been long continued which is afforded by 

 the prevailing thinness of the loess deposits is supported by the indica- 

 tions as to the slight effect which wind erosion has as yet had upon the 

 topographic details of this district. That this form of erosion though 

 now active has not been long continued is clearly indicated by the pre- 

 vailing sharpness of the.water-scul[)turing of the lesser reliefs, such as 

 are to be found liere and there in the now arid valleys. Thus at a point 

 where Alder gulch opens into the broad and extremely arid valley of 

 Ivuby rivi'r a tletached boss of gneissic rock, rising to the height of 40 or 

 50 feet al)ove the level of the neighboring valley, in a jiosition where it 

 is much ex})osed to the action of the blown dust, is very scantily worn. 

 It requires, indeetl, close observation to detect any evidence of cutting 

 action such as dustdaden winds etfect. Like conditions may be ob- 

 served at many other points in this and neighboring valleys. So, too, 

 with tlie pel)bles of the mesa-like benches resulting from the erosion of 

 the ancient lacustrine deposits. These fragments are as yet but little 

 worn by the wind-blown sands, and this wearing appears to be limited 



