SUMMARY OF CONCLUSIONS 251 



3. The essential absence of loess deposits interstratified with gravels 

 indicates that the existing arid period is, as regards modern geological 

 time, quite exceptional, and, furthermore, that since the period when this 

 region was the recipient of extensive showers of volcanic ashes (perhaps 

 the middle Tertiar}'^ ?) the district has been too humid for the formation 

 of wind-blown deposits. 



4. At present the process of desiccation, except so far as it is effected 

 by the burning of the forests, appears to be arrested — a fact which leads 

 to the conclusion that the climatal changes that led to the existing arid 

 conditions are no longer in the process of development. 



5. The process of wind erosion here, as elsewhere under like conditions, 

 serves to produce and transport a great amount of fine detritus to the posi- 

 tion where it may be readily taken up by the rivers and sent on its way 

 to the sea; the result of this action being at once to increase the effi- 

 ciency of the river work, to greatly extend the area of effective erosion, 

 and to overburden the streams with fine sediment. Incidentally it serves 

 to diminish the down-cutting of the upper parts of a river system, those 

 just below the true torrents, in which the arid conditions most occur by 

 overloading the water with the transportable material. Thus in an arid 

 mountain region, such as we are considering, there is an upper zone of 

 true torrent work, and below it, occupying more than half of the whole 

 region, a valley zone where the erosion is of a very contrasted nature, 

 being evenly and widely distributed with an aerial delivery of the 

 detritus to the streams. 



6. The facts suggest that the total amount of erosion in an arid region 

 where the work of the wind is effective, in a measure found in the exist- 

 ing condition in central Montana, may be much greater than would be 

 the case in a region of far larger rainfall, where the assault on the rocks 

 is practically limited to the torrents ; also that the distribution of the 

 erosion in these contrasted climatal states may be very diverse — in the 

 humid aspect of the country greatest in the highlands ; in the arid, 

 most effective in the broad lower valleys. 



7. The effect of this wind erosion is rapidly to degrade the quality of 

 the soil there by removing the fine material which constitutes the greater 

 part of its value. 



It is hardly necessary to suggest the desirability of inquiries having 



