AND (' 0 A L - AI EA8URES O F IOWA. 



125 



level, near our encampment of the 9th of September, as designated on the Chart. 

 Not far from this place we encountered two granite boulders, the first large erratics 

 we observed in our ascent of the Des Moines. A chalybeate spring issues from 

 under the coal-banks. 



A short distance from this section, ridges one hundred to two hundred feet high, 

 composed chiefly of sand, gravel, and drift deposits set in, concealing the carboni- 

 ferous stata, so that they only appear at long intervals. It was not until after 

 passing the Boone Fork, that an opportunity was offered of again inspecting the 

 rocks. 



A short distance above the mouth of Brushy Creek, micaceous sandstones, of the 

 coal formation, are exposed, at an elevation of from fifteen to twenty feet ; and two 

 miles further, on the right bank, a bluish gray clay is seen near the water's edge, 

 charged with fragments of argillaceous iron ore, together with an impure gray 

 calcareous rock, and loose pieces of soft sandstone ; but the exact order of super- 

 position cannot be seen, by reason of the thick vegetation and deep alluvion. 



Five miles above Brushy Creek, argillaceous shale, with nodules of ironstone, is 

 overlaid by slaty sandstone, which latter is about fifteen feet above the water-level. 

 (See Section No. 61, D.) About a quarter of a mile beyond, hard, black, bitumi- 

 nous shale, splitting into sheets, rises from beneath the water-level, and soon attains 

 an elevation of from fifteen to twenty feet, covered by erratic deposits, which con- 

 tinue to accumulate in proceeding towards the north. The shales enclose large 

 angular masses of compact black calcareous rock, which may answer for an hydraulic 

 cement. 



About half to three-quarters of a mile above this locality, a conspicuous boulder 

 of porphyritic granite lies in the river, near the left bank. Ten miles above the 

 Boone Fork of the Des Moines, solid ledges of sandstone, containing vegetable 

 impressions, and embracing some thin, interpolated layers of conglomerate, present 

 themselves to view. These sandstones gradually increase in thickness on approach- 

 ing the great easterly bend of the " Burnt Woods District," where the current of 

 the Des Moines strikes mural escarpments of thirty to fifty feet on alternate sides, 

 as it is deflected from one high point of land to another. Twenty to twenty-five 

 miles above the Boone Fork, they even attain the height of one hundred and ten 

 feet. (See Section No. 62, D.) 



Near the termination of these bluffs, just below our encampment of the 5th 

 September, in sight of the Burnt Woods, there is a good deal of hydrated oxide of 

 iron, but rather too much impregnated with sandy particles to be of practical value. 

 The sandstones, with some shaly intermixtures, continue within sight of Lott's 

 Rapids, about five miles below the point where the north and south line, between 

 Ranges 28 and 29, crosses the Des Moines. Here, limestones can be seen at a low 

 stage of water, covered with a multitude of boulders, which fill the channel ; and 

 these so obstruct the navigation, that it was with difficulty we succeeded in floating 

 the bark canoe containing our provisions and camp equipage, over the shoals, 

 although the river was high. 



It seems as if an accumulation of drift had taken place, from some local cause, 

 such, perhaps, as the stranding of an iceberg loaded with erratics ; since above and 



