112 IOWA STUDIES IN NATURAL HISTORY 



So far they have been found at Seneca, Wisconsin, on the 

 flat summit of the south quartzite range near Devil's Lake, 

 Wisconsin, on the summit plain in the south portion of the 

 Sparta quadrangle north of Cashton in Wisconsin, in the 

 Tomah quadrangle, Wisconsin, at Iron Hill near Waukon, 

 near Church, and near Elon in Iowa, and in various portions 

 of the Driftless Area in Minnesota. In all these places the 

 gravel is thick enough to form a measurable deposit and at 

 Senaca and Waukon the thickness is as great as 35 feet. The 

 deposit occupies summit positions in the topography, which 

 position represents the Dodgeville plain in each case. In 

 addition to these localities where the gravel is in place, there 

 are many places in Iowa and Illinois and probably in Wis- 

 consin and Minnesota, where scattered pebbles which have 

 been derived from the deposit are found at all levels. How- 

 ever, there is no place known where the gravel lies in its 

 original position at levels below the Dodgeville plain. At 

 Devil's Lake they are associated with potholes in the sum- 

 mit surface. It is not believed that these patches of gravel 

 are remnants of a formation which once covered the entire 

 Driftless Area, but that the gravels were deposited only 

 along stream courses. 



There can be no doubt that these gravels are of fluvial 

 origin. The pebbles range in size from a small fraction of 

 one inch to three or four inches in diameter. The smaller 

 ones are rounded and highly polished and seem to have been 

 carried far, or at least to have undergone transportation 

 for a long time. The large ones are more irregular and 

 some of them seem hardly to have been transported at all. 

 At Seneca, Waukon and Elon, the gravel deposits are dis- 

 tributed in crescent shaped areas resembling the curves of 

 streams. 



The writer has broken hundreds of the pebbles and has 

 yet to find one composed of anything but silica. Most of 

 them are chert, but some are white quartz, some are almost 

 black, and some have the color and appearance of jasper 

 and chalcedony. They are known to include nothing which 

 could not have been derived from the pre-Cambrian and 

 Paleozoic formations which originally covered the Driftless 



