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Rocks

Sedimentary rocks

Sandstone cliffs in Talampaya National ParkSandstone cliffs in Talampaya National ParkSedimentary rocks, such as sandstone and mudstone, are made from sand, gravel, mud and other fragments of rock that result from erosion. Limestone and chalk, also types of sedimentary rock, is made of the remains of tiny marine plants and animals that lived millions of years ago. The fragments settle in layers in lakes, rivers and seas. As more layers settle on top of each other, the particles are compressed and cemented into sedimentary rock.


A river lays down sediments.A river lays down sediments.

Sediments

Sedimentary rocks are made up of rock fragments. These fragments come from other rocks that have been broken down by the weather over thousands of years. Removed by wind, water or ice (a process known as erosion), the fragments may settle on a stream bed, say, or at the bottom of the sea. They are called sediments. Sand, silt, gravel, mud, even the remains of living creatures, are all kinds of sediments. If they lie undisturbed for a few million years, the grains will cement together to form rocks. Sandstone, shale and limestone are all sedimentary rocks.
A close-up view of glacial sedimentsA close-up view of glacial sediments

Ammonite fossils in limestoneAmmonite fossils in limestone

Limestone

Although much of the Earth's land area is covered by sedimentary rocks, they form only about 8% of the total volume of the Earth's crust.

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