Page 6--Sedimentary Rocks

Sedimentary rocks, the most common rock at the surface in Kansas, can be found throughout the state. They may be formed from other rocks that were worn down by erosion into small pieces, such as sand and gravel, called sediment. A solid rock may be formed if the sediment is covered by great thicknesses of other sediment and rocks and pressed together, or if it is cemented together by minerals.

Organic material, which is anything that was once alive, may also be pressed together into a solid mass. Fossils of shells and tiny microscopic plants and animals, left behind when ancient seas dried up, are found in much of the Kansas chalk and limestone. Other common sedimentary rocks in Kansas are clay, shale, bentonite, silt, siltstone, sand, sandstone, dolomite, salt, and coal.

Clay, Shale, and Bentonite

Clay is composed of very fine particles eroded from rocks and minerals. These particles are so finely worn that they can only be seen with a microscope. Clays may be a variety of colors-white, gray, black, red, yellow, tan, or green-and are often mixed with larger particles of other sediment such as sand and pebbles. Bricks, dishes, and other ceramic products are made from clay, which is molded and then hardened by heat.

Figure 15. Bricks made of clay pave this sidewalk in Lawrence.

When clay and silt are compacted into a solid rock, it is called shale. Shale erodes easily into clay when exposed in outcrops and roadcuts. Like clay, shale can be many different colors and is common in Kansas and throughout the world. It is used to make bricks and as an ingredient in cement.

Bentonite is a type of clay formed from altered volcanic ash. Most types of bentonite swell when they absorb water. Deposits of bentonite have been found in several locations in western Kansas.

Silt and Siltstone

Silt consists of particles larger than clay particles but smaller than sand particles. It is deposited by wind and water. Loess (pronounced lus) is a windblown silt found in many areas of Kansas. Thick loess deposits occur in northeast Kansas where rocks and gravel were ground down by glaciers and water. Later the dried mud was picked up by the wind. Much of it settled near the margins of the glaciers. But loess also covers much of the surface in western Kansas where it was spread around by ferocious dust storms.

When silt is compacted and cemented together, it forms a rock called siltstone, which is found in eastern Kansas.

Sand and Sandstone

Sand particles vary in size and can easily be seen without a microscope. Sand often contains eroded particles of rocks and minerals carried downstream by rivers and creeks. Two minerals, quartz and feldspar, are commonly found in sand.

Sand deposits are widespread in the state, especially along streams and river valleys and in old river deposits. Sand hills also cover large areas of south-central and southwest Kansas.

Figure 16. Wind and water have shaped this sand dune near the Arkansas River in Kearny County in southwest Kansas.

When sand is cemented together, it is called sandstone. Sandstones in Kansas range in color from a light tan to brown to reddish-orange to bright orange. The different colors are caused by a variety of minerals and impurities. Sandstone deposits are found throughout Kansas, and buildings made of sandstone blocks can be found in areas where it is most abundant.

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Kansas Geological Survey
Placed online Feb. 1, 1996
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