254 GENERAL GEOLOGY. 



These various folds in the strata of Iowa, thus far mentioned 

 and referred to, all evidently took place subsequent to the 

 deposition of the latest strata of Carboniferous age, and 

 before any of those of Cretaceous age were deposited; 

 probably at the close of palseozoic time. This is inferred 

 from the fact that all our palaeozoic strata appear to have 

 partaken of those disturbances, but the Cretaceous (mesozoic) 

 strata do not.* 



The broad fold or elevation of the strata which passes 

 down through the center of the State from north to south, 

 becomes so flattened in southern and southwestern Iowa that 

 in the latter region, especially the Upper coal-measure strata, 

 occupy the whole surface, and over nearly the whole region 

 that it occupies, its southerly dip is almost exactly coincident 

 with the southerly slope of the streams. This fact will be 

 definitely referred to in the following remarks. 



A section of the rocks exposed in the valley of Middle 

 river, near Winterset, in Madison county, is given on a 

 previous page in connection with the description of the Upper 

 coal-measure formation, to which the reader is again referred. 

 Although this locality is only about a dozen miles from the 

 extreme border of its outcrop, the section is the most complete 

 one of this formation to be found exposed at any one 

 locality in the State, and is believed to comprise in a few of 

 its upper members the highest beds of the Upper coal- 

 measures to be found within its limits. Therefore, that 

 section is frequently referred to as a typical one for the 

 formation. 



It is almost necessary to infer the gradual thickening of all 

 the coal-measure strata to the southwestward, to some extent 

 at least, and we have the evidence that although a few of the 

 upper members of the Winterset section disappear, the lower 

 members of the same do thicken in that direction, so that the 



* The strata of Cretaceous age in Western Iowa, although they seem to occupy a 

 perfect level are found by railroad levelings and other data to have also been slightly 

 inclined since their deposition. Their unconforrnability, however, upon the palaeozoic 

 strata is unmistakable. 



