Rocks and minerals of the Osage Cuestas

Sandstone. Like sand, sandstone is made up largely of quartz grains, which are held together by some natural cement such as calcium carbonate, iron oxide, or silica. It is a common sedimentary rock in Kansas. In eastern Kansas, sandstone is often interbedded with shale and limestone. It also occurs as channel deposits, cutting through older deposits of shale and limestone. The Tonganoxie Sandstone Member of the Stranger Formation, which crops out in Franklin, Douglas, and Leavenworth counties, is an example of a channel deposit; it was deposited in a large river valley about 300 million years ago. Today the Tonganoxie is an important aquifer. Another sandstone in this region, the Bandera Quarry Shale Member, is quarried in Bourbon County, near the town of Redfield. This sandstone, part of the Bandera Shale, separates easily along natural bedding planes and is used as flagstone for walkways and veneer.

Shale. Another sedimentary rock common in Kansas, shale is composed of hardened, compacted clay or silt that commonly breaks along bedding planes. Its particles are too small to be seen without a microscope. Shales erode easily, and most are soft enough to be cut with a knife. Though usually gray, shale can be black, green, red, or buff. In eastern Kansas, shale has been used for making bricks. When heated, shale changes to the familiar brick-red color. Black, platy shales found in Labette, Crawford, Bourbon, Douglas, Linn, and Neosho counties contain large amounts of organic matter. Called oil shale, this rock can yield oil when distilled and small slivers can be ignited with a match.

Limestone. Perhaps the most common rock found at the surface in Kansas, limestone is composed mostly of calcite (calcium carbonate, CaCO₃), which comes from the shells of marine organisms that accumulated on the sea floors. The limestones that crop out in the Osage Cuestas contain a variety of fossils, evidence of the abundant life that populated the seas during the Pennsylvanian subperiod. The Beil Limestone Member in Osage County, for example, contains brachiopods, bryozoans, fusulinids, mollusks, and cornucopia-shaped horn coral. Some limestones in this region contain ooids, small spheres of calcium carbonate cemented around a grain of sand, shell fragment, or other nucleus. Oolitic limestones can be seen in Johnson, Miami, Osage, Linn, Bourbon, and Labette counties and near the towns of Independence and Cherryvale in Montgomery County.

The limestones and shales in the Osage Cuestas were deposited in shallow seas that lapped onto this area between about 290 and 310 million years ago. The type of rock exposed in a particular outcrop gives geologists information about the environment at that spot millions of years ago. As material was eroded from the land and carried into the sea by streams, the coarsest material was deposited near the shore while finer sediments were carried farther seaward. Although there are exceptions to this general rule, a bed of sandstone usually indicates deposition on or very near shore, and layers of shale (which were once mud) indicate deposition a little farther from shore. Limestone was deposited during periods of decreased erosion or in areas where sandstones and shales were not being deposited. The marine organisms associated with many limestones indicate the environment was tropical and the water was relatively shallow and clear.

A number of years ago, geologists who studied the Pennsylvanian and Permian strata of the midcontinent noted that sandstone, shale, and limestone reoccured in predictable order upward through a vertical sequence of beds. In its simplest form, this recurring order was sandstone, shale, limestone, shale, and sandstone. Geologists theorized that such a sequence represented a single advance and withdrawal of the shoreline of an ancient sea past a certain geographic point. Dozens of such cycles were soon recognized in the strata of the Osage Cuestas region. From this and other evidence, geologists deduced that the Pennsylvanian and Permian seas were relatively shallow and that their bottoms were nearly flat or only gently sloping. Because of this, small changes in sea level caused the shoreline to move great distances back and forth across the region that is now Kansas.

Lamproites. Almost all rocks at the surface in Kansas are sedimentary, but this region is also known for exposures of igneous rocks in Woodson and Wilson counties. These rocks, which geologists called lamproites, crop out in sill-like masses about one mile long and several hundred yards wide. Lamproites are dark-colored igneous rocks rich in potassium and magnesium, which formed from the cooling of molten magma. Geologists refer to these rocks as intrusive because they were forced into other rocks below the surface of the earth. These lamproites formed during the Cretaceous Period, about 100 million years ago. They contain irregular pieces of hardened or altered shale. When the hot, molten igneous rock came in contact with the Paleozoic shale, pieces of the shale broke off and fell into the liquid mass.

Coal. As in the Cherokee Lowlands, coal mining played an important role in this region. Coal (which consists chiefly of carbon compounds derived from plants and plant debris that have been compacted into firm, brittle rocks) occurs throughout the Osage Cuestas. It was extensively mined in Osage County. Between 1885 and 1892, Osage County had 118 coal mines, which employed more than 2,200 people and produced almost 400,000 tons of coal.