124 GEORGE F. KAY AND J. NEWTON PEARCE 



certain colloidal properties. In fact recent uncompleted work 

 by Mr. L. B. Miller^ gives evidence which not only supports the 

 idea that these two zones differ chiefly in respect to differences in 

 colloidal properties but also strongly confirms the theory that the 

 gumbotil is directly related to the drift. It has already been 

 stated that colloidal properties are primarily properties of the 

 surface. A given colloid material of different degrees of sub- 

 division will adsorb var5dng amounts of a given substance, and 

 the amounts adsorbed by a given mass of the colloid will be approxi- 

 mately proportional to the specific colloidal surface. Assuming 

 that hygroscopic water adsorbed by clays may be taken as a 

 measure of coUoidality and of surface development, Miller has 

 determined the amount of hygroscopic water taken up by each 

 of these clays at 25.° He has found that, beginning with the 

 original drift material the specific surface increases gradually, but at 

 an increasing rate upward to the gumbotil. He has also determined 

 the "total-water capacity," that is, the amount of water per gram 

 which is just sufficient to cause the clays to "run." This likewise is 

 greatest for the gumbotil in any drift, and it decreases gradually 

 downward through the lower layers. The high "total- water 

 capacity" of the gumbotil accounts for the ease with which it 

 slides in exposed cuts. 



SUMMARY 



The aim of this paper has been to show by field and laboratory 

 evidence that the gumbotils on Nebraskan, Kansan, and Illinoian 

 glacial tills are the result chiefly of chemical weathering of drift. 

 Thus far no distinctive evidence has been found in Iowa to indicate 

 that the bowlder clay from which gumbotil is thought to have been 

 derived differed to any great extent from typical bowlder clay. 

 In the case of the lowan and Wisconsin glacial tiUs, which are too 

 young to have had a gumbotil developed on them, the till at and 

 near the surface does not appear to differ in any important respects 

 from the till which is deeper below the surface. In this connection 

 it should be stated that Mr. E. W. Shaw, as a result of his studies 

 of the Illinoian drift in southern IlKnois, the Kansan drift in 



' L. B. Miller, The Colloidal Properties of Clays. 



