Sedimentary Rock Types IV
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Mudstone is the same as sandstone except the particles of rock that make up the mudstone are too small to be called sand. As the name sounds, the rock used to be mud that was buried and hardened into rock. Siltstone is made from even smaller particles than mudstone. Fossils can also be found in mudstone and siltstone. These types of rocks are sometimes called slate. In the United States, the national monument named Fossil Butte is near mudstone and siltstone formations that hold hundreds of fish fossils.
Like “ripple marked” sandstone, mudstone can have marks made by water on it. Sometimes when mud hardens the surface cracks; these cracks can be preserved and harden into rock. Even more rare, the marks made by falling raindrops will be preserved in the rock. I have found the mud crack-marked rocks, but not the rain-marked rocks. I'll have to keep looking.
Biochemical sedimentary rocks are formed from the debris of life. For example, limestone is formed from out of decayed animal shells. Animals use calcium to form their shells. After the animal dies, the shell falls apart and the calcium combines with other elements and minerals and hardens into rock. Fossils are very common in this type of rock. I have found fossil trilobites in this rock. Another example of this type of rock is coquina. Coquina is formed from pieces of seashells cemented together. My geology professor had some, it was really neat to look at. Coal and chert are also common biochemical sedimentary rocks.
Chemical sedimentary rocks are formed when water evaporates and leaves behind minerals that harden into rock. A great example of this kind of rock is salt. So remember when you're eating salt, you're eating a rock. It's a rock called halite.
Work at a salt mine. Salt mineral.
The other category of sedimentary rock is very rare, usually caused by an impact like an asteroid. The asteroid hits so hard it breaks up rocks and fuses them together into a new sedimentary rock. Another example is hot rock spewed from a volcano, but not hot enough to melt the rock it touches; it hardens around other bits of rock forming a new sedimentary rock. Like I said, this is very rare.
Sedimentary rocks only form about 8 percent of the rocks on Earth that cover the other types of rocks like a thin coat of paint. Even though they are only a tiny percentage of the rocks on Earth, they are very important. They tell us a lot about the history of life on earth because sedimentary rocks are the only type of rock that can hold fossils and they are formed in layers with the oldest rocks on the bottom and the newest rocks on the top.