116 PROCEEDINGS OE THE WASHINGTON MEETING 



differently by different authors, the most recently published view being that 

 of Tilton, who considers the material to have been formed, in the main, dur- 

 ing the retreating stages of the Kansan iee. To this gumbo and other ma- 

 terials which he considers to be related in age to the gumbo he has given the 

 name Dallas deposits. 



Detailed field studies which are still in progress in southern Iowa seem to 

 warrant the author in making a preliminary statement involving some inter- 

 pretations which differ from those previously advanced. 



(1) The surface of the Kansan drift, after the Kansan ice withdrew, was, 

 according to present evidence, a ground moraine plain, which, from the main 

 divide between the Mississippi and Missouri rivers, sloped gently to the south- 

 east and south toward the Mississippi and to the southwestward toward the 

 Missouri. This drift plain was so situated topographically that weathering 

 agents were very effective, but erosion was slight. As a result of the weather- 

 ing during an exceedingly long time a grayish, tenacious, thoroughly leached, 

 and non-laminated joint clay, which has been named gumbo, was developed to 

 a maximum thickness of more than 20 feet. This gumbo contains only a few 

 pebbles, which are almost wholly siliceous, and grades downward into yellowish 

 and chocolate-colored Kansan drift from 3 to 7 feet in thickness, in many 

 places with numerous pebbles, few, if any, of which are calcareous. This 

 oxidized but non-calcareous drift, in turn, merges into unleached drift, oxidized 

 yellowish for several feet, below which is the normal unleached and unoxidized 

 dark-grayish to bluish-black Kansan drift. The gumbo is believed, therefore, 

 to be essentially the result of the thorough chemical weathering of the Kansan 

 drift; but, subordinately, other factors, such as the wind, freezing and thawing, 

 burrowing of animals, slope wash, etcetera, have undoubtedly contributed to 

 its formation. The Kansan drift which has been changed to gumbo may have 

 differed somewhat from the normal Kansan drift that lies below the gumbo. 



(2) After the gumbo plain had been developed by weathering processes on 

 the Kansan drift plain, diastrophic movements seem to have occurred, the 

 plain having been elevated to such an extent that erosion became effective and 

 valleys began to be cut into the gumbo plain. Erosion of the gumbo plain 

 progressed to such an extent that some valleys were cut to a depth of more 

 than 150 feet before grade was reached and a mature topography was devel- 

 oped. Only remnants of the original gumbo plain remain, the most conspicu- 

 ous of these being flat, poorly drained areas, known as tabular divides. Where 

 creep and slumping have occurred the gumbo, in places, may be found on 

 slopes at an elevation several feet below the level of the gumbo plain. The 

 tabular divides are more prevalent east of a line drawn north and south 

 through south-central Iowa than west of such a line. In the southwestern 

 part of the State the Kansan gumbo, which is in situ, is found only where the 

 divides, which are no longer distinctly tabular, retain the level of the former 

 gumbo plain. 



(3) While there is, in places, loess of eolian origin on the Kansan drift of 

 southern Iowa, much of the material which has been described as loess is 

 thought to be not of eolian origin, but to be related more or less closely to 

 the gumbo. The upper few feet of the Kansan gumbo, which is now limited 

 to the tabular divides and divides closely related to tabular divides, is a fine- 

 grained, loesslike, joint clay, in which, if diligent search is made, it is possible 



