Facts about Clastic Sedimentary Rocks

Clastic sedimentary rocks are composed of other rocks that have broken down or been eroded away. This means that they are fragments of “clasts” of other previously existing rocks. For the most part, that means they are from either igneous, metamorphic, or sedimentary rocks. Once these rocks break apart, flowing water, wind, moving ice, or other natural causes deposit the particles in a place where they can begin to form.

Cement or mineral crystals then begin to gather and hold the small sediment particles together. Over time, the particles will clump together to form rocks where they can then be identified by their grain size, material composition and their texture.

Checking the grain size is the first step in identifying a clastic rock, and a magnifying glass is needed to see the shape of the crystals. If the grain is larger than 2 mm wide, then it is classified as having gravel clasts. However, if the grain is less than 2 mm and larger than 0.0625 mm, then it is identified as being composed of sand. Finally, if the grain is less than 0.0625 mm, it falls under the mud category.

If the grain is gravel-sized and upon closer inspection the clasts have rounded corners, then the rock is conglomerate. The rock is only considered a breccia if the clasts are gravel-sized and have angular corners.

When the grains are determined to be within the sand category, then the rock is called a sandstone. The three most common types of sandstones include lithic sandstone (or litharenite), quartz and arkosic stones. If the sandstone is composed of rock fragments, including but not limited to limestone, shale, rhyolite or other sandstone particles, then it is called a lithic sandstone. For it to be identified as quartz, the main component must then be quartz. For the stone to be arkosic, feldspar particles must be able to be identified. Because arkosic sandstones contain a high amount of quartz, the feldspar then becomes a huge indicator to tell the two apart.

Finally, there is the mud group, which has grains that are so small (less than 0.0625 mm) that it must be identified through its components rather than the grain shape. Two major categories of mud clasts include slit and clay stones. If the particles are between 0.0625 mm and 0.0039 mm, than the coarser grains are called a slit or slitstone. When the particles are smaller than 0.0039 mm, the rock is made of clay and called a claystone.

Sometimes determining what kind of clastic rock you have can be difficult, so knowing where it came from can help. Perhaps it was found in a muddy environment, such as a swamp or at the bottom of a lake; then you probably have a mud clast. If it came from the beach or desert, then it is mostly likely a sandstone. Finally, if it was discovered at the bottom of a stream than it could be either a conglomerate or a breccia. Clastic sedimentary rocks are generally easy to find and simple to classify if you know what to look for.