Clay is one of the most common earth substances in Kansas. It is a very fine grained material that can be molded into shapes and can be heated or baked into hard, resistant forms that have many uses. The particles in a clay deposit are so small (less than 1/256 mm in diameter) that they cannot be seen without a microscope. Formed by weathering and breaking down of solid rocks, these particles may then be carried to some quiet body of water such as a lake, pond or the sea, where they settle to the bottom. Clay particles are made of several types of minerals, most of which are small platy flakes. The deposits formed may have almost any color - white, gray, black, red, yellow, buff or green. Many clay deposits contain impurities such as sand, calcium carbonate and iron minerals.

Clays and shales are used in making bricks, tiles, pottery, chemical ware, furnace linings and lightweight concrete aggregates. In Kansas, the best deposits of refractory clays (those that can withstand firing at high temperatures) are found in rocks of Cretaceous age in Washington, Clay, Cloud, Lincoln, Ottawa, Ellsworth and other central Kansas counties. Thousands of tons of the clays are used each year in the manufacture of light-colored face brick, with smaller amounts used in making pottery.

Bentonite is a clay formed by chemical alteration of volcanic ash. When water is added, bentonite may swell to as much as 15 times its original bulk and forms a milky cloud in the water. Most Kansas bentonite swell, when wet to 3 times their original volume. Some bentonites can be identified by their waxy or soapy appearance. Many deposits are found in western Kansas, the thickest in Phillips County. Very thin layers can be seen in McAllaster Buttes in Logan County. A thin layer occurs in places above the volcanic ash in the Calvert ash mine in Norton County. Other thin deposits are interbedded with the chalks and chalky shales in western Kansas.

Underclay is a clay that occurs under a coal bed or under a coal horizon. This clay is generally characterized by a lack of bedding and commonly contains fossilized roots of plants and other carbonaceous material. Underclay is present under many of the coal beds of southeastern Kansas.

In Cherokee County it is utilitized in the manufacture of buff brick. Some of the underclays are suitable for firing at high temperatures.


Shale is a hardened, compacted clay or silty clay that commonly breaks along bedding planes some of which are no thicker than paper.

Shales are easily eroded or worn away. (When shales weather, they form clays or muds)

The best exposures are found beneath ledges of harder more resistant rocks such as limestone and sandstones. Most shales are soft enough to be cut with a knife and can be very brittle. They are usually gray, but black, green, red or buff shales are also common. Many contain nodules of pyrite, selenite (gypsum) or phosphate minerals. Shale and clay make up about 80% of the sedimentary rocks of the Earth's crust. Dark-gray to black Cretaceous shales, hundreds of feet thick, are common in the west-central part of the state. Much of the shale found in Pennsylvanian rocks is interbedded with layers of limestone.

In Labette and Neosho counties (and other eastern counties) there are several black, platy shales that contain larger amounts of organic matter. Some are so rich in this material, that thin sliver may be set on fire with a match (they are a form of oil shale). Shale in eastern Kansas has been used for making bricks. When heated it color changes to "brick-red". Shale is also used mixed with limestone for making Portland cement.

Shale sometimes is confused with the metamorphic rock slate because of the color and ability to break along bedding planes. There are no naturally occuring slate deposits in Kansas.

Table 2 - Particle Size
Sedimentary Rocks



Rounded, Subrounded, Subangular

> 256mm Boulder Boulder gravel, Boulder conglomerate

256 - 64mm Cobble Cobble gravel, cobble conglomerate

64 - 4mm Pebble Pebble gravel, pebble conglomerate
4 - 2 mm Granule Granule gravel  
2 - 1/16 mm Sand Sand, Sandstone Grit (1/2 - 1 mm)
1/16 - 1/256mm Silt Silt, Siltstone  
< 1/256 mm Clay Clay, Shale