274 G. R. Keyes — Redrock Sandstone of Marion Co., Iowa. 



being the same. The full thickness of the sandstone is not 

 represented in the cut The upper limit is very uneven and 



paved every where with rounded water- worn bowlders and peb- 

 bles, derived from the sandstone itself. A gray fire-clay covers 

 this pavement and upon it rests a coal bed having a thickness 

 of six feet centrally, but rapidly thinning out laterally in both 

 directions to a very unimportant, scarcely recognizable, bitum- 

 inous seam. Northward, or at right angles to the face of the 

 section, the coal is thicker. Superimposed upon the coal are 

 drab and ash-colored, clayey shales, having an exposed thick- 

 ness of thirty feet, but which are manifestly much more ex- 

 tensive. From a consideration of this section, then, it is clear 

 that before the superimposing coal seam was formed the vast 

 sand bed had been raised above the surface of the waters, con- 

 solidated, and was then subjected to considerable denudation. 

 In a small gorge or ravine, excavated in the sandstone, the car- 

 bonaceous material was deposited as the land. T^as again being 

 submerged. Immediately to the north of +~ ft section repre- 

 sented in the figure (which faces the south', che corrasion was 

 much more extensive, as is shown by the rapid inclination of 

 the axis of the gorge in that direction ; so that the section is 

 actually across a tributary ravine opening into a large basin in 

 which the coal is now mined in large quantities. The infer- 

 ence is, then, that the abrupt disappearance of the vast bed of 

 sandstone in such a short distance as half a mile above the 

 quarry, where it has an exposure of more than one hundred 

 feet, is not due wholly to the inclination of the stratum, but is 

 the result of great erosion in that direction, previous to the 

 deposition of the shales and clays ; and that the massive sand- 

 stone really formed a bare hill of considerable height against 

 which the subsequent deposits were laid, when the conditions 

 for such a change occurred. 



Three miles down the river from the Eedrock quarry is 

 another instructive exposure. A small but deep ravine divides 

 the section. On the left is the concretionary limestone — the 

 last outcrop of the St. Louis in Central Iowa to be noted in the 

 ascent of the DesMoines river. At this place it rises in a low 

 arch about fifteen feet above low-water. Overlying it are 

 marly and somewhat sandy clays or shales which have a 

 vertical exposure of sixteen feet. The strata dip 10° to the 



