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Clastic Sedimentary Rocks: Definition, Types & Examples

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  • 0:00 What Are Sedimentary Rocks?
  • 1:16 Forming Clastic…
  • 2:04 Types of Clastic…
  • 3:57 Lesson Summary
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Lesson Transcript
Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

Clastic sedimentary rocks are the geologic record of places like streams, deserts, and flood plains. This lesson covers the wide variety of clastic sedimentary rocks and what they tell us about geologic history.

What Are Sedimentary Rocks?

New geology students are often bewildered by geologists' fascination with rocks. However, when geologists are looking at rocks in an area, they see the change of desert to swamp lands or the rise and fall of rivers, all over hundreds of millions of years! Sedimentary rocks are a strong indicator of how the landscape looked in the past. Sedimentary rocks are really like telescopes that allow us to look into the very distant past.

grand canyon

Check out the image above. Every layer of the Grand Canyon marks a different sedimentary rock that is tied to a different ancient environment. Sedimentary rocks are rocks formed by the accumulation of sediments. These are either clastic, biochemical, or chemical sedimentary rocks. Clastic sedimentary rocks are formed by an accumulation of weathered fragments of other rocks that have hardened with a matrix material. Take a look at the picture below.

conglomerate microscope image

This picture of a sedimentary rock taken with a microscope clearly shows the weathered rounded clasts and the matrix material. This lesson will also provide some examples of the depositional environmental, or the historic environment where the rock was originally formed.

Forming Clastic Sedimentary Rocks

To make a clastic sedimentary rock takes multiple steps. First, an existing rock must go through weathering which is the mechanical breaking down of rock, normally by flowing water or ice formation. These weathered fragments must be transported to a separate location where they can form a new rock. Sediments are typically transported by moving water or wind. Then, the rock is normally flooded, allowing finer sediments or precipitates, or minerals formed out of water, to harden around the sediments and form a hardened rock. This process of matrix material filling in between the sediments is called cementation. As our sediments are buried by more material, the pressure on the sediments and matrix increases until it reaches a point where a hardened sedimentary rock forms.

Types of Clastic Sedimentary Rocks

Clastic sedimentary rocks are most commonly classified by the size of the sediments, called grain size. The clastic rocks with the largest grain size are conglomerates and breccias. Conglomerates are coarse-grained clastic rocks formed of rounded cobbles that are most often formed from riverbed deposits. If you've ever waded barefoot into a river, you've noticed the smooth cobbles at the bottom of the riverbed. These cobbles eventually form into conglomerate rocks.

Breccias are the angular sediment cousins to conglomerates. Breccias are also coarse-grained but have angular clasts that are formed by more drastic events like rock-falls and earthquake deposits that are cemented together.

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