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 little pieces of other rocks called sediment. Mineral crystals called cement hold the sediment together.

There are many different types of clastic sedimentary rocks. To figure out which type of rock you have, you will need to figure out the answers to these three questions:

• How big are the sediments?
• Are all of the sediments about the same size?
• Are the sediments rounded or angular in shape?

You will probably need to use a magnifying glass to see the little sediment grains.

Different types of sedimentary rocks form in different environments. For instance, sandstone, a sedimentary rock made of sand grains, may form in a beach 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 sediments at the bottom of a stream.

Making a clastic sedimentary rock is a four-step process.

• Step 1: Breaking Rocks Apart
• Step 2: Sediments on the Move!
• Step 3: Sediments Settling Down
• Step 4: Turning a Pile of Sediment into Solid Rock