118 PROCEEDINGS OP THE WASHINGTON MEETING 



terval much longer than some of us have beeu inclined to think heretofore, 

 especially since the development of the gumbo appears to have taken place 

 largely, if not wholly, before the dissection of the Kansan drift plain. It 

 seems to be confined to the flat uplands and does not mantle the eroded slopes. 

 In any case it clearly marks a stratigraphic horizon, the top of the Kansan till. 



The point to which I wish particularly to call attention is the relation of 

 this super-Kansan gumbo to tbe lowan drift. For a number of years past 

 there has been considerable question in the minds of some of the geologists 

 as to whether there really is a post-Kansari drift in northeastern Iowa. This 

 was because of difference of interpretation of some of the phenomena. It was 

 finally decided that the Federal Survey should cooperate with the Iowa 

 Geological Survey in a critical review of the evidence in the held. I was 

 assigned to this work with Morris M. Leighton of the Iowa Survey as assistant, 

 and we spent the held seasons of 1914 and 1915 in the investigation. It is a 

 pleasure to be able to announce that we have reached the conclusion that there 

 really is good evidence of the presence of a post-Kansan drift sheet in that 

 area ; so that we are in the main in agreement with the late Doctor Calvin in 

 regard to the lowan drift. 



We found the area much less markedly eroded than the Kansan drift area. 

 The topography is of a peculiar type. We have called it a mantled, mature- 

 erosion topography — that is, there are there the large patterns of maturely 

 developed dendritic drainage systems — but most of the minor details have ap- 

 parently been smootbed out. It is what one would expect if an ice-sheet were 

 to spread over the maturely dissected topography of the Kansan drift area, 

 wearing away the spurs, smoothing out the irregularities, partly filling the 

 valleys, and mantling the whole with a rather thin sheet of till. This is indeed 

 just what seems to have occurred. The slopes are very largely long, low, 

 smooth, and, excepting close to the main streams, almost wholly uncut by 

 lateral drainage lines. The minor valleys are shallow, open swales with 

 smoothly rounded bottoms, where the streams flow in shallow trenches. There 

 is a notable absence of the V-shaped cross-profiles seen in the Kansan area. 



The uppermost drift throughout the lowan drift area is a highly calcareous 

 till. It is more modified by weathering than the Wisconsin, more so indeed 

 than one would infer from some of the published descriptions, yet not nearly 

 so much changed as the Kansan drift ; neither is it so much modified as the 

 Illinoian drift. The famous big granite boulders, although not entirely con- 

 fined to the lowan drift area, are notably abundant there and seem to be 

 particularly characteristic of this drift deposit. 



Now, the particular interest in regard to the super-Kansan gumbo is that 

 it affords a new line of evidence which tends to corroborate the other data. 

 Many new exposures have recently been afforded by grading for country roads, 

 electric interurban railroad lines, and the Chicago, Milwaukee and Saint Paul 

 Railroad ; also we made about 250 test borings with an auger S feet in length. 

 We have found numerous exposures of the super-Kansan gumbo, or a deposit 

 exactly like it, in the higher parts of the lowan area, those parts particularly 

 where remnants of the original Kansan drift plain are likely to have been 

 preserved. In several places there is a black carbonaceous bed at the top, 

 apparently an old soil, and overlying this a thin deposit of later till — the lowan 

 till. Where this till is less than 4 or 5 feet thick, its limestone pebbles and 



