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.
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Courtesy of Larry Fellows and the Arizona Geological Survey
Clastic Rocks
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.
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