66 SOIL PHYSICS AND MANAGEMENT 



comparatively recent geologic time. As a result of this action, large 

 deposits of volcanic dust are found in Washington, Oregon, Idaho, 

 Montana, Wyoming, and Nebraska. In the latter state the deposit 

 varies from 4 to 30 feet in thickness, while in some of the north- 

 western states the deposit is much deeper. To give some idea of the 

 amount of dust that is transported by the wind during a volcanic 

 eruption, and the distance to which it may be carried, it is said that 

 the dust from a volcano in Nicaragua was distributed by the wind 

 over 1,500,000 square miles and that ashes from Krakatoa fell to 

 a depth of several inches at a distance of a thousand miles from 

 the volcano. 



QUESTIONS 



1. Give some examples of dust falls. 



2. Give classes of wind-laid material. 



3. Where are sand dunes found and what is the source of the sand? 



4. What is the shape of sand dunes? How may they vary from this? 



5. How is sand movement stopped on the shores? 



6. How may fixed dunes be changed to wandering onea? 



7. What is a " blowout " ? 



8. What special advantage does beach grass have tor preventing drifting of 



sand? 



9. Where are loess deposits found? 



10. Give reasons for believing that loess is a wind deposit. 



11. Do dust particles carry a film of air? 



12. If so, what is the effect of this on the specific gravity of the particle? 



13. What are some of the characteristics of loess? 



14. How does it compare with dust? 



15. Give characteristics of adobe. 



16. Where is it found? 



1?. How extensive are deposits of volcanic dust? 



REFERENCES 



1 UddeU, J. A., Popular Science Monthly, September, 188(5. 

 "Coffey, G. N., Journal of Geology, vol. xvii, No. 8, 1909. 



* Merrill, G. P., Rocks, Rock- Weathering and Soils, p. 319. 

 4 Op. Cit., p. 318. 



Subaerial Deposits of North America, Geol. Mag., August, 1889. 



General Reference. Free, E. E., Bulletin 68, Bureau of Soils, U. S. 

 D. A., The Movement of Soil Material by the Wind, with Bibliography. 



