GEOLOGY OF SOUTHWESTERN IOWA. 307 



erosive action, both that which occurred during the Glacial 

 epoch, and that which resulted from ordinary meteorological 

 agencies in connection with the drainage while subsequently 

 forming the valleys. By a glance at the following sections of 

 strata in this county, it will be seen that those referred to the 

 Middle coal-measures, are nearly all of such a character as to 

 have yielded readily to the erosive action to which they have 

 been subjected in common with others that elsewhere occu- 

 pied the surface, and the resulting inequalities of surface are 

 consequently more gentle than they are where firmer mate- 

 rials have been subjected to the same process. It will 

 also be seen that the strata referred to the Upper coal- 

 measures are in great part limestones, which are so firm that 

 they have yielded far less to the mechanical erosion of the 

 drift agencies than others have, but have nevertheless, yielded 

 completely to the action of the streams. This, however, is 

 probably accomplished by a process of solution, to a great 

 extent, rather than by the mere mechanical attrition produced 

 by running water. 



One may obtain an opportunity to appreciate quite fully 

 the difference in the surface-characteristics of the two forma- 

 tions mentioned, by going up the valley of Middle river, 

 starting from the east boundary of the county. For the first 

 eight or ten miles he finds the valley-sides gently sloping to 

 the stream on each side; but he then comes suddenly upon 

 the border of the Upper coal-measure formation, as is shown 

 by the bold bluff exposures of its limestone strata in the 

 valley-sides. Within a few miles further he finds himself in 

 a narrow valley, more than two hundred feet deep, and 

 bordered on each side by limestone bluffs. This character 

 continues along the same valley as far as the west boundary 

 of the county. 



The deep erosion of these streams into the limestone strata 

 sometimes ends quite abruptly, as is seen in the valley of 

 Clanton's fork of Middle river, which, at the village of Peru, 

 it is upward of two hundred feet deep, having cut its way 

 down through more than one hundred feet in vertical 



