142 S. CALVIN PRESENT PHASE OF PLEISTOCENE IN IOWA 



jacent Kansan. The great body of the Illinoian, where unaltered, is 

 yellow in color; the Kansan, as already noted, is blue. The Illinoian 

 surface shows practically no erosion, except at the margin or along the 

 larger stream valleys; over more than three-fourths of its area there is 

 little or none of the sculpturing effect of surface drainage. At its mar- 

 gin, or near the larger streams which have been successful in keeping 

 their valleys scoured out, there is some water carving, but the narrow, 

 rain-cut gulches are steep, and they terminate at most only a few miles 

 back from the foot of the grade, in the uninvaded plateau. All the char- 

 acteristics of the Illinoian may be studied between Durant and Daven- 

 port, or between Columbus Junction and Morning Sun or Mediapolis. 

 Weathering, as expressed by oxidation, hydration, and kaolinization of 

 feldspars, affected the deposit to a depth of 3 or 4 feet. Compare now 

 with the Kansan. In the older drift the changes due to weathering, what- 

 ever the names employed to denote processes of alteration, have gone down 

 to depths varying from 12 to 30 or 40 feet, and that in a stiff er and more 

 impervious clay than the Illinoian. The whole surface over hundreds of 

 square miles is carved by storm waters into well rounded ridges and deep, 

 wide-open ravines. The lateral ravines on the side slopes of the larger 

 drainage valleys have very gentle gradients as compared with those in the 

 Illinoian, and they reach back into the interstream areas, up to the sum- 

 mits of the rounded divides. Excepting in a few limited localities where 

 there are very wide or relatively low spaces between the larger systems 

 of drainage, there is none of the original drift plateau left. In his paper 

 in the Proceedings of the Iowa Academy of Sciences, volume V, Leverett 

 cites evidence to show that quite an amount of erosion had probably taken 

 place in the surface of the Kansan before the Illinoian drift was depos- 

 ited. The amount of erosion indicated is much more than has been 

 effected in the general surface of the younger of the two drifts we are 

 comparing since the disappearance of the Illinoian ice. But even if we 

 disregard this particular bit of evidence of probable pre-Illinoian erosion 

 of the Kansan, it is still true that, under similar conditions as to prox- 

 imity to the main drainage courses and altitude above them, the amount 

 of erosion in the Kansan is very much more than twice as great as in the 

 Illinoian. On the basis of erosion alone we are justified in concluding 

 that the Yarmouth interval was longer than all post-Illinoian time. 



Considering the effects of weathering and general alteration, the evi- 

 dence leads to the same conclusion. The first few feet below the surface 

 should undergo alteration rapidly. This zone is exposed more directly 

 to the atmosphere, to storm waters, to thawing and freezing, to the chem- 

 ical reactions of carbon dioxide, humic acids, and other products of de- 

 caying vegetation ; to the direct and indirect effects of burrowing animals, 



