REMARKABLE UNCONFORMITY AT REDROCK. 287 



the Lower Coal Measures there are several breaks in deposition of equal or 

 even greater importance. 



As to the relations of the upper and lower Carboniferous rocks along the line 

 of the general section, the most instructive exposures are at Harvey and Elk 

 cliff, two miles below the village of Redrock. At the former locality the 

 limestone is overlain by a few feet of ash-colored, highly fossiliferous marl, 

 whose upper surface everywhere has been disturbed by running water, while 

 in many places gullies have been made down to the hard limestone. Upon 

 this the Lower Coal Measure shales have been deposited. In some places at 

 a subsequent time these have also been removed in part, and the whole is 

 covered by drift material or loss. The Lower Coal Measure shales that 

 immediately overlie the St. Louis rocks carry at least two workable seams 

 of coal and supply characteristic upper Carboniferous fossils. The thickness 

 of these shales is at least seventy-five feet before reaching the base of the 

 Redrock sandstone. At Elk cliff, however, this sandstone apparently rests 

 directly upon a low anticlinal fold of the St. Louis limestone, which at this 

 place rises about fifteen feet above low water in the Des Moines river. The 

 inference is that the limestone at this place formed a low island at the be- 

 ginning of the later Carboniferous, and continued as such until the Redrock 

 sand bed had commenced to accumulate. 



The upper surface of the Redrock sandstone, however, offers the most re- 

 markable illustration of unconformity in the region. At Redrock bluff, and 

 above, the evidence in support of this statement is admirably exposed. 

 Everywhere the sandrock has been worn and channelled, often to a depth 

 of more than one hundred feet. In these narrow gorges and ravines, coal 

 and shales have been laid down upon the rounded water-worn pebbles and 

 small bowlders of the sandstone itself, which cover the old eroded surface. 



A third noteworthy unconformity exists at the old town of Bennington, 

 in the northwestern corner of Marion county. The inclined strata have 

 been abruptly worn away on a line nearly parallel to the horizon ; and after- 

 ward there were deposited the materials of a shaly sandstone, which now 

 shows no noticeable dip. It is possible that a double unconformity is repre- 

 sented at this place, and that the prominent coal seam rests on the worn 

 surface of the compact massive sandstone, which is visible for a distance of 

 ten feet above low water. In lithological characters this sandrock is in all 

 respects identical with the Redrock sandstone, and probably it is really the 

 summit of an eroded elevation of the great sand stratum. 



There is also considerable evidence of the existence of minor unconform- 

 ities, in connection with several of the coal seams, along the line of the 

 section ; but in the absence of fresh artificial exposures their extent has not 

 yet been satisfactorily determined. 



