Sedimentary Rock Types III
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Clastic sedimentary rocks are sedimentary rocks formed out of broken down bits of rocks. There are several kinds of clastic sedimentary rocks; they are categorized by the size and sometimes the shape of the pieces that make up the rock. The kinds of clastic sedimentary rocks are called: Conglomerates, breccias, sandstone, mudstone, siltstone.
Conglomerates are made from bits of rounded gravel that were deposited by the agent, usually water. Over time the wet pile of sediments becomes so heavy that the gravel gets compacted together, and minerals in the water cement the bits of gravel together. The only difference between conglomerates and breccias is: Conglomerates are made from rounded gravel and breccias are make from angular gravel.
Conglomurate rock Breccia rock
Sandstone is basically the same. Piles of sand are piled up by water or wind and over time the piles get so heavy the sand is pressed together and cemented by minerals found in ground water. There are some interesting things that can be found in sandstone. Fossils are often found in sandstone. Also, have you ever been to the beach or a lake and looked out into the water and noticed that the sand is piled up in ripples by the motion of the waves? Sometimes the sand will get buried and the ripple pattern is preserved. Over time the sand will harden into rock and the ripple marks will still be in the rock. I have found this “ripple marked” sandstone. Somewhere in my house I still have some.
Sometimes sand dunes will become so large they harden into rock preserving the hump shape of the sand dune. There is a very famous fossil that was found in China in one of these preserved sand dunes. The fossil is called the “fighting dinosaurs.” Paleontologists think a velociraptor and protoceratops were fighting and were trapped by a collapsing sand dune.
The Fighting Dinosaurs. Photo: David Clark