SHEETFLOOD AND EOLIAN EROSION 81 



Locating engineers of railroads have yet to learn how to properly make 

 surveys across the arid country. 



In August, 1906, one of the trains carrying some of the members of 

 the International Geological Congress to the city of Mexico passed, in the 

 state of Chihuahua, through five fioodsheets in a single day. One of 

 them, on a plain with a noticeable grade, appeared, as far as eye could 

 reach, like a lake with shrubbery everywhere growing out of it. When 

 closer inspection was made it was soon found that the waters were in 

 rapid motion down the slope, which happened to be toward the railway 

 track. Part of the floodsheet reached the track before the train suc- 

 ceeded in getting beyond its effects and buried the roadbed in mud. The 

 train following was delayed hours, and it was afterward learned that the 

 mud covered the tracks in places to depths of more than 2 feet. The 

 roadbed for a distance of more than 3 miles was completely lost to view. 



McGee, I think, greatly overestimates the actual extent of sbeetflood 

 action as a destructive agency of erosion. It is of great significance that 

 he recognizes throughout the Sonoran district which he describes, and 

 far beyond its borders, that the intermont plains all have rock-floors, in 

 most places only very thinly veneered with sands and silts. He ascribes 

 this condition entirely to sheetflood work. Sheetflood action is probably 

 secondary to eolian action in these regions. Sheetflood effects are very 

 local and of sporadic nature; wind action is universal and constant. A 

 floodsheet may visit a given locality only once a year; the wind blows 

 over the plains all the time. 



EOLIAN EROSION IN ARID REGIONS 



In the arid regions the wind is probably not only the most potent of 

 the gradative agencies, but its efficiency is greater than all other geologic 

 processes combined. The cooperation of sheetflood activities in making 

 the plains is important; without extensive wash the present smoothness 

 of the plains would be impossible. 



The main activity of the wind is manifestly degradational in character. 

 The constructional effects are local and relatively unimportant. It is 

 thought that in the arid regions of western America the wind has been 

 a general leveling agent the importance of which has been little suspected. 



Throughout the dry regions "dust storms" are violent and frequent. 

 During their progress their effects in producing personal discomforts 

 have commonly blinded all, even trained scientists, to their real geologic 

 significance. While these dust storms last, and even for several days 

 afterward, the air is so filled with fine soil that it is often impossible to 



