Notes on the Redrock Sandstone

The sandstone of Redrock, in Marion County, Iowa, has long attracted popular attention. The bright vermillion cliffs rise to a height of one hundred to one hundred and fifty feet above the water surface of the Des Moines River. The red coloration of the rock, however, is (?). The formation has a known geographic extent of at least twenty miles; and probably stretches out much farther. At Redrock Cliff the stone is massive for the most part, but rather soft and thin-bedded above. At this place it is a very fine grained and homogeneous sand rock, some portions even affording excellent material for grindstones. But southeastward, and at Elk Bluff, two miles below, the sandstone passes into a fine-grained, ferrugineous conglomerate. The dip is everywhere to the south and west; and at a short distance above the quarry, a short distance above the village, the inclination is very considerable. A mile beyond, the sandstone has disappeared completely and the section shows only shales and clays. The space between the latter exposure and the last known outcrop of the sandstone is perhaps half a mile, the interval being hidden by quaternary deposits down to the water level. The abrupt change in the lithological character of the rocks in so short a distance has been mentioned by Owen and by Worthen; but the true explanation is entirely different from the suppositions of those writers