Clastic Sedimentary Rocks

                                                         river rocks

Clastic sedimentary rocks may first be classified according to their grain size. Clay-sized particles are too small to be seen with a microscope. 

Rock formed from clay-size particles are called shale. Silt-sized particles are visible with a microscope. 

Rock formed from these are called siltstone. Sand-sized grains are visible to the naked eye and range from 1/16 mm to 2 mm. 

Sand is further subdivided into very fine, fine, medium, coarse, and very coarse. Rock formed from these are called sandstone. "Gravel"-sized grains range from > 2 mm granules to very large boulders. 

Rock containing these large size particles are called conglomerate and are typically very poorly sorted (e.g., they may contain, sand, gravel, and boulders all in one rock). If the gravel particles are little weathered and are still angular (un-rounded) the rock is called breccia. 


http://www.columbia.edu/~vjd1/sed_rx.htm