486 EUGENE WESLEY SHAW 



streams; a deposit due to subsidence; a deposit due to climatic 

 change; and in southwestern Wisconsin a closely related but pre- 

 dominantly stream-laid deposit has been attributed to glacial 

 floods and deposits in the Mississippi Valley. 



The clay is not glacial drift, for it contains no stones and little 

 sand; and much of it lies outside the glacial boundary. More- 

 over, it is found only in the lowest places and its upper surface is 

 horizontal without regard to the underlying surface of hard rock. 

 It is not loess, for it fills all depressions up to certain altitudes and 

 is not found at higher positions. Its thickness and others of the 

 characters already described show that it is not a normal flood- 

 plain deposit. It could scarcely be a simple back-water deposit 

 from glacial floods without the help of a valley train, because that 

 would require that the rivers have a sustained depth of about two 

 hundred feet for the thousands of years it must have taken the clay 

 to accumulate. A subsidence of the surface might lead to the 

 development of a few bodies of clay having the shape and arrange- 

 ment of those under discussion, but warping so complex as to cause 

 the regular arrangement and shape of so many bodies of clay would 

 be inconceivable. Nor could the deposits have been produced by 

 climatic change, for such deposits slope down stream and these 

 are horizontal. Finally, the limited up-stream extent of the clay, 

 the fineness of the material, the horizontality of the surface, and 

 the fact that the clay abuts against thick bodies of coarser material 

 on the large rivers, indicate that most of the clay accumulated in 

 lakes produced by valley fillings, the master drainage lines of 

 the region. In order to understand the cause and history of the 

 lakes it is therefore necessary to look into the history of the large 

 rivers. 



Valley filling on the Mississippi and Ohio. — The deposits on the 

 Mississippi and Ohio consist principally of sand, but there is con- 

 siderable gravel and silt, the gravel being more abundant at the 

 base and the silt at the top. Most of the material lies below extreme 

 high- water stage, and hence the surface forms a flood-plain, but 

 here and there bodies of sand and gravel stand about 30 feet above 

 the reach of high water, the upper surface in such places forming 

 a terrace at the altitude of the valley filling on near-by tributaries. 



