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Sedimentary Rocks


How are they formed?Sedimentary_Sandstone_Rock
Sedimentary rocks are formed when eroded fragments of old rocks and dead organisms settle (usually in seas or rivers) to form a sediment. Over millions of years, layers of sediment build up and are buried one on top of the other. They are compressed, and their weight squeezes out the water. Eventually the pieces of rock in the sediment become cemented together to form sedimentary rocks.

What are they like?
Sedimentary rocks have a layered appearance. They may contain fossil remains of animals and plants that were trapped as the rocks formed. These fossils can be used to date the rock.

Limestonesandstone and mudstone are examples of sedimentary rocks.


Sedimentary Rock:  formed when sediments are pressed & cemented together

Weathering:  when rocks are broken down into smaller fragments

Erosion:  when rock fragments are moved by some force of nature

Deposition:  when rock fragments are laid down in a new location

Formed by erosion.  Sediments are moved from one place to another, and deposited in layers, with the older ones on the bottom.  The layers become compacted and cemented together to form sedimentary rock.

Strata:  layers of rock

Stratification:  the process in which sedimentary rocks are arranged in layers


Clastic – made of fragments of rock cemented together with calcite or quartz (ex.:  breccia)

Chemical sedimentary – minerals crystallize out of solution to become rock (ex.:  limestone)

Organic sedimentary – remains of plants and animals (ex.:  coal)

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