GEOLOGY OF SOUTHWESTERN IOWA. 319 



and near the valleys. As one travels along the valleys of 

 the creeks he not unfrequently meets with spaces of conside- 

 rable length, sometimes amonnting to a quarter of a mile, 

 where the water is without apparent current. At such places, 

 it often has a width many times that of the same creek where 

 it flows with the ordinary current, and he can hardly believe 

 that it is not arrested by a mill-dam, or some other artificial 

 obstruction. 



These, however, are natural features and are caused by the 

 almost perfectly horizontal position of the strata and their 

 alternations of clayey shale and limestone, before mentioned. 

 The strata are so nearly level that when the current has 

 swept out the soft shale, the firm layer of limestone beneath 

 it serves as an unyielding bed to the stream which will 

 not allow of the further deepening of its bed, and is only 

 swept clean by every flood. When that stratum of limestone 

 is finally passed over by the creek, and another soft layer 

 is reached and swept out, another limestone floor is left 

 which supports its still pond of water of greater or less 

 extent as before described. These features are particularly 

 characteristic of Elk creek and both of its branches. 



Again, along some of the larger valleys of the county, 

 although no rocks may appear in view, the presence there 

 beneath the surface of the valley-sides of the alternating 

 strata before mentioned, is apparent in the alternate widening 

 and narrowing, and by other characters of the flood-plains. 

 These are sometimes quite out of ordinary proportion to the 

 size of the streams. Their soil in such cases is usually 

 clayey, instead of sandy, as is commonly the case with the 

 soil of flood-plains, the clay being evidently derived from the 

 clayey strata before mentioned, and the presence of the latter 

 is also the cause of the variable width of the flood-plains. 

 These peculiar features of the valleys although occasionally 

 recognised in those of the creeks or smaller streams of the 

 county, are quite conspicuous along the valley of Grand 

 river, particularly in that part of it which lies between the 

 mouth of Elk creek and the State boundary. Here the 



