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.

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 Lake Scott State Park 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.


Buchanan, R., and McCauley, J. R., 2010, Roadside Kansas: A Traveler's Guide to its Geology and Landmarks: Lawrence, Kansas, University Press of Kansas, 392 p.

GeoFacts: High Plains (pdf)