High Plains—Rocks and Minerals

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Download fact sheet on the rocks and minerals of the High Plains.

Loess. Loess covers much of the uplands in northern and western Kansas, concealing many of the rocks near the surface. Loess is a finely ground silt that is deposited by the wind. In the High Plains of Kansas, loess was deposited by the wind during the Ice Ages of the past million years. This finely ground silt was formed as glaciers advanced over the continent, pulverizing rocks and sediments in their path. When the glaciers melted, this silt was deposited on the floodplains by streams coming from the melting ice sheet. Geologists think that temperature differences between the snow-covered regions to the north and the bare ground to the south may have created large differences in atmospheric pressure, which produced strong winds capable of moving large amounts of silt a considerable distance.

More then 90 percent of the soil in Thomas, Sherman, Cheyenne, Greeley, Wichita, Scott, Hamilton, and Lane counties has developed in loess deposits. In some places the loess has been eroded away by streams. The resulting draws and canyons have extremely steep sides (loess maintains a nearly vertical face without slumping or caving in). Along the Arikaree River in Cheyenne County, canyons carved into thick loess deposits form a rugged landscape called the Arikaree Breaks.

color photo of rugged landscape with yucca in foreground

Arikaree Breaks, Cheyenne County.

Sandstone. A common sedimentary rock, sandstone is made up largely of quartz grains that are held together by some natural cement such as calcium carbonate, iron oxide, or silica. In the High Plains, the most common rock in the Ogallala Formation is a porous sandstone made up of quartz and feldspar grains that are cemented by very fine-grained calcium carbonate. Because these rocks look like concrete, they are known locally as mortar beds. Ogallala mortar beds crop out in the bluffs around Scott County State Lake and near the town of Cedar Bluffs in Decatur County.

Another kind of sandstone--a hard, dense, gray-green rock--also occurs in some parts of the Ogallala Formation, especially in southern Phillips County, but also in Graham, Hodgeman, Ness, Norton, Rawlins, Rooks, and Smith counties. The sandstone is cemented with opal. This opaline sandstone is occasionally used as a building material.

Opal. Opal is found in the Ogallala Formation in Clark, Ellis, Logan, Ness, and Rawlins counties. Opal consists of silicon dioxide (SiO2), like quartz, plus an indefinite amount of water. It never forms as crystals and cannot be scratched by a knife, though it is slightly softer than quartz.

Kansas opals are not the precious variety. The opals from the Ogallala may be colorless, white, or gray and are found with a white, cherty calcareous rock. Some of it is called "moss opal" because it contains an impurity (manganese oxide) that forms dark, branching deposits like small mosses in the opal. Moss opal has been found in Trego and Wallace counties.

Jurassic Rocks. The Jurassic Period occurred between 208 and 144 million years ago, just before the Cretaceous Period. During the Jurassic, sandstones and shales were deposited over the western one-third of Kansas. These Jurassic formations were then covered during the Cretaceous and are found only in the subsurface, except at a few locations in the southwest corner of the state. One of these, known as Point of Rocks, overlooks the Cimarron River in west-central Morton County. This outcrop of Jurassic rocks is capped by the younger rocks of the Ogallala Formation.

color photo of outcrop up close

Jurassic outcrop at Point of Rocks, Morton County.

Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.

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