Rock Topics

Classification of Sedimentary Rocks (Part 1)

Sedimentary rocks are classified based on their texture and composition.  Detrital sediment has a clastic (broken) texture.  Chemical and organic sediments have a non-clastic texture, and are classified based solely on their composition.

Clastic Texture

Clastic rocks are named for their grain size and shape. 

Coarse-grained (>2 mm) rocks with rounded grains are called conglomerate.  If the coarse-grains are angular, the rocks are called breccia.

Conglomerate and Breccia
Left: conglomerate; right: sedimentary breccia (Photographs by Stan Johnson)

MapConglomerates (left) are coarse-grained sedimentary rocks with rounded grains.  This sample is from Loudoun County.  Breccias (right) are coarse-grained sedimentary rocks with angular grains.  This sample is from Culpeper County.  Note quarters for scale.

Medium-grained (1/16 mm to 2 mm) rocks are usually called sandstone.  Medium-grained rocks can be composed of quartz grains, feldspar grains (arkose), or rock fragments and clay (graywacke).

Sandstone (Photograph by Parvinder Sethi)

Medium-sized grains in clastic sedimentary rocks form sandstone. This sample is a quartz sandstone.

Fine-grained (<1/16 mm) rocks are generally called mudrocks.  They include siltstone and shale.

ShaleShale from Grayson County (Photograph by Parvinder Sethi)

MapFine-grained sediments usually form shale.  Shale is a detrital sedimentary rock that splits readily along thin bedding planes.  This sample is from Grayson County in the Mount Rogers area.