PAPERS ON GEOLOGY 105 



SOURCE OF THE LOESS MATERIALS 



Two sources have been suggested for the materials of the 

 loess. Some geologists have assumed that it has been blown 

 by the winds from the arid regions of the Great Plains, while 

 others think it has been derived from the rock flour of glacial 

 till. The former theory is discredited by the following facts: 



1. The small size of the particles of the surface soil of the 

 arid regions. Merrill 4 says "the particles of adobe or surface 

 soil of the Great plains vary in size from those too small for 

 measurement, up to .08 millimeters in diameter. It will be 

 seen from the former table that both the maximum and mini- 

 mum size of the particles of the loess are larger than those 

 of adobe, which is the opposite of what would be expected if 

 the latter was the source of the loess. 



2. The constituents of adobe contain a much larger propor- 

 tion of calcium carbonate and of mineral matter derived from 

 sedimentary rocks than does the loess. 



3. The peculiar distribution of the loess in relation to the 

 drift deposits of the upper Mississippi Valley. Chamberlin 

 says, "The constitution of the loess of the Mississippi Valley 

 taken with its two distributive relationships proves it to have 

 a special origin from the glacial drift." 



4. Adobe soil is much more variable in its chemical com- 

 position than the loess, as appears in the following comparative 

 table of analyses in which columns 1 to 5 show the analyses 

 of loess from different localities, and columns 6 and 7 are 

 those of adobe soil. 



The peculiar relations of the loess to the various drift sheets 

 and to the streams of the glaciated region and the rseem- 

 blance of the loess in freshness and mineral composition to the 

 finer parts of the till, furnish convincing evidence that the 

 source oi the loess material was glacial drift. It also seems 

 reasonable to assume that the belts of greatest thickness of the 

 loess are nearest the immediate sources of supply. If this is 

 true the main immediate sources of the loess are, 1. river flood 

 plains, and 2, the Iowan drift sheet. 



4. George P. Merrill: Rocks, Rock Weathering and Soils; p. 321, 1916. 



