The Nebraskan Drift

The bedrock exposures of Northwestern Iowa occur chiefly in the slopes of the Big Sioux and the Missouri valleys along the west boundary of the state. Along the west line of Plymouth and Northern Woodbury counties, there are many small outcrops, along the west line of Southern Sioux County there are a few, and in Lyon County are two or three outcrops in the very northwest corner of the state. Away from these valleys only two small exposures of bedrock have been reported in Northwestern Iowa. Most of these outcrops are in steep valley slopes, evidently gouged away by the glacial action of a later (Wisconsin) drift.

Two widely separated divisions of the geologic column are represented by the bedrock outcrops in the Big Sioux Valley. The few exposures in Northwestern Lyon County are of the Proterozoic, or quart-like rocks, while those of the counties to the south are of Cretaceous rocks. In the northwestern corner of Lyon County there are two exposures of what is known as the Sioux quartzite. One of these lies near the Minnesota line, and the rock stands as a ridge twenty feet high and about a quarter of a mile long. The other outcrop is a small valley two miles to the east. These two exposures in Iowa lie on the south border of a large area of quartzite, which extends north to Flandreau, South Dakota, a distance of forty-five miles, and has its eastern limit at Redstone Ridge, Cottonwood County, Minnesota, and its western limit at Mitchell, South Dakota. The rock is well exposed at Rowena, South Dakota, just north of the outcrops on the Iowa side, in and around Sioux Falls, and at many other places in Eastern South Dakota and Southwestern Minnesota. The area within which the Sioux quartzite directly underlies the drift in Northwestern Iowa cannot be definitely outlined. The Sioux quartzite and other ancient rocks of its geologic age form a basic foundation for Northwestern Iowa. Their upper surface dips southward from the outcrops at an altitude of more than 1,400 feet above sea level, at Sioux City.

This basic stone of Northwestern Iowa, the Sioux quartzite, is a very hard vitreous rock, varying in color from pink to red. It consists of rounded quartz sand grains, so firmly cemented with silica that the whole resembles a mass of quartz.


Source: Northwestern Iowa Its History and Traditions 1804-1926 by Arthur F. Allen Volume 1.