116 ILLINOIS ACADEMY OF SCIENCE 



imum loess deposition. Leverett 1 thinks that, since loess oc- 

 curs in a few places between the Kansan and Illinoian drift 

 sheets as well as more generally above the Illinoian and be- 

 low the Wisconsin till, there must have been two periods of 

 aridity in this region during Pleistocene time. 



In his recent paper on the "Solar Hypothesis of Climatic 

 Changes," Huntington 2 has assumed that arid conditions pre- 

 vailed in the loess area of the Mississippi Valley during each 

 of the stages of glaciation. 



However, the following facts make it very improbable that 

 anything like desert or even semi-arid conditions could have 

 accompanied the deposition of the loess : 



1. The regularity, or very gradual change, in the thickness 

 of the loess over the uplands. Wind deposits in arid regions 

 are notoriously dune-like and irregular. The level, loess cov- 

 ered prairies of the Illinoian drift plain present none of the 

 irregular features characteristic of wind deposits in arid re- 

 gions where a dense cover of vegetation did not control the 

 permanent lodgment of the material. 



2. The presence throughout the thicker loess deposits of 

 shells of species of land snails that live at present on woodland 

 hills in the same regions, indicates that similar conditions of 

 habitat had existed throughout the time of accumulation of 

 the loess deposits. 



3. The absence of glacio-fluvial or other water laid de- 

 posits associated with the Iowan drift is also evidence opposed 

 to an arid or semi-arid climate during that time, for under arid 

 conditions the rains would be concentrated during a short sea- 

 son of the year, making the streams more effective in the 

 transportation and deposition of coarse debris. 



4. The uniformly fine grain of the deposit, which was de- 

 rived from sources rich in coarser sand material, also precludes 

 the possibility of arid conditions in which the winds had any 

 greater efficiency than at present. The texture of the main 

 portion of the loess deposits is as fine as that of the uppermost 

 part which was presumably deposited under present climatal 

 conditions in more recent time, and in any exposure there is 

 generally no line or zone in which any change in the texture 

 of the materials of the deposit can be detected. 



In the loess deposits bordering the Embarrass and Wabash 

 rivers in Crawford and Lawrence counties, Illinois, a bed of 



1. Frank Leverett: Weathering and Erosion as Time Measures. Am. Jour, of 

 Science, Vol. XXVII, May, 1909, p. 361. 



2. Ellsworth Huntington : The Solar Hypothesis of Climatic Changes. Bui. Geol. 

 of Am., vol. XXV, No. 4, 1914, p. 577. 



