596 SAMUEL CALVIN 



Devonian age." The collector may gather Lime Creek fossils in 

 the pastures and cultivated fields. The peculiarities of these 

 areas, so far as they are seen in Cerro Gordo County, are noted in 

 Iowa Geological Survey, VIII, 175, where, years ago, the statement 

 quoted was published. 



With the exception of the sub-Aftonian, or Nebraskan, which 

 does not give character anywhere to parts of the glaciated territory 

 large enough for mapping, each drift sheet has its characteristic 

 topography which prevails over the major part of its area, and each 

 has its exceptional phases which affect but a small percentage of 

 its surface. There is a broad belt of typical Iowan between the 

 Wapsipinicon and the Cedar, north of Walker. With the exception 

 of a narrow strip west of the larger river, the broad area between 

 the anomalous Cedar and Flood creeks in Mitchell, Floyd, and 

 Franklin counties is as strikingly level, uneroded, and free from 

 drainage courses as much of the typical Wisconsin, and in some 

 places it is also quite as calcareous. Flood Creek is simply a prairie 

 stream that scarcely breaks the monotony of the plain that extends 

 from the Cedar to the Shell Rock; through its entire course north 

 of Nora Springs, even the Shell Rock flows in a young, shallow 

 trench cut in the otherwise unbroken Iowan plain. Areas such as 

 these — scores of miles in length and width, with scarcely a drainage 

 trench outside the channels of the larger streams — illustrate the 

 real type of erosion in the Iowan; these show the topography of a 

 real Iowan drift plain, and it is scarcely necessary to add that that 

 topography is characteristic of youth. 



The typical bowlders of the Iowan are coarse feldspathic 

 granites in no way remarkable for their power to resist the destruc- 

 tive agencies of weathering, and yet very little decomposition has 

 taken place amongst them since they were left exposed at the time 

 of the retreating Iowan ice. In some way, either before or during 

 transportation, many of the bowlders were fractured, and in such 

 cases the angles are still comparatively sharp (Fig.. 10), while bowl- 

 ders of corresponding texture in the Buchanan gravels or in the 

 weathered zone of the Kansan drift are completely decayed. 

 Topography, bowlders, and stratigraphic position all unite in 

 support of the theses: 



