Clastic Rocks

Cathedral Rock in Arizona is made of a clastic rock called sandstone. . These rocks are more than 250 million years old and are now being weathered and eroded away by the creek. Click on image for full size (37K JPG) Courtesy of Larry Fellows and the Arizona Geological Survey

Clastic sedimentary rocks are made up of fragments of other rocks called sediment. Larger pieces are called grains or clasts. Small, muddy pieces are called matrix. In a clastic sedimentary rock, the clasts and the matrix are held together with mineral crystals called cement.

There are many different types of clastic sedimentary rocks. They can be identified based on:

• The size of clasts (Are they mud, sand, or pebbles?)
• The range of clast sizes (Are they all of similar size?)
• The shape of the clasts (Are they rounded or angular?)

The types of sedimentary rocks that are formed in an area depend on the environment where they were deposited. For instance, sandstone, a sedimentary rock made of sand grains, may form in a beach environment or desert sand dunes. Shale, a sedimentary rock made of mud and clay, may form in a swamp, the bottom of a lake, or some other muddy environment. Conglomerate, a sedimentary rock make of gravel and sand, may form from the sediment at the bottom of a stream.

Making a clastic sedimentary rock is a four step process.

• Step 1: Weathering (Destroying Rock)
• Step 2: Erosion and Transport (Sediments on the Move)
• Step 3: Deposition (Sediments Settling Down)
• Step 4: Lithification (From Pile of Sand to Solid Rock)