308 COUNTY AND REGIONAL GEOLOGY. 



thickness of limestone, yet within five miles further np the same 

 valley, the stream is a mere prairie creek, with gently sloping 

 valley- sides, composed of drift alone; and yet there are no 

 definite falls in the whole course of the stream. This deep 

 erosion of the limestone strata is quite characteristic of all 

 the streams that traverse the Upper coal-measure formation 

 in this county, as is well shown in the valleys of South and 

 North rivers, as well as in those of the two rivers before 

 referred to. This is doubtless largely due to the fact that 

 their drainage is in an opposite direction from the dip of the 

 strata, and that the limstones are intercalated and underlaid 

 by softer strata, so that they are readily undermined by the 

 action of running water. 



Geology. Madison county, as already mentioned, is under- 

 laid by the Upper and Middle coal-measure formations. The 

 Upper occupies much of the largest part of the area, and 

 is much the most conspicuous in the characters it presents 

 for observation. Indeed the most important and interesting 

 exposures of this formation to be found in the State, are 

 found in Madison county. The Middle coal-measures occupy 

 the northeastern township of the county exclusively, as well 

 as the one also that lies immediately south of it, besides 

 considerable portions of the townships adjoining them upon 

 the west and south. Besides this they are to be occasionally 

 seen in the valleys of the larger streams as far westward 

 as the middle of the county. 



The sections which follow, will give an idea of the character 

 of the strata that underlie the county as far as they are 

 accessible from the surface. The section measured in the 

 valley sides of Middle river near Winterset, is so complete 

 and extensive that the lithograph illustration of it has been 

 used for purposes of general comparison in the chapter on 

 the Upper coal-measure formation. The reader is referred to 

 that as well as to the description of it which is repeated here. 



Section at Winterset. 



16. Thin bedded, yellowish-gray limestone 1 foot. 



15. Fossiliferous, marly clay 4 feet. 



