Why is sedimentary rock layers

Figure 4. In this lesson, you will learn about sedimentary rocks like sandstone, how they form, how they are classified, and how people often use sedimentary rocks. Most sediments settle out of water Figure 4. For example, running water in rivers carries huge amounts of sediments. The river dumps these sediments along its banks and at the end of its course. When sediments settle out of water, they form horizontal layers. One layer at a time is put down. Each new layer forms on top of the layers that were already there.

Thus, each layer in a sedimentary rock is younger than the layer under it and older than the layer over it. When the sediments harden, the layers are preserved. In large outcrops of sedimentary rocks, you can often see layers that show the position and order in which the original sediment layers were deposited.

Scientists can figure out the relative ages of layers by knowing that older ones are on the bottom and younger ones are on top.

There are many different types of environments where sedimentary rocks form. Some places where you can see large deposits of sediments today include a beach and a desert. Sediments are also continuously depositing at the bottom of the ocean and in lakes, ponds, rivers, marshes and swamps. Avalanches produce large unsorted piles of sediment. The deposited rocks build up in layers, called sediments.

This process is called sedimentation. The weight of the sediments on top squashes the sediments at the bottom. This is called compaction.

The water is squeezed out from between the pieces of rock and crystals of different salts form. The crystals stick the pieces of rock together. This process is called cementation. These processes eventually make a type of rock called sedimentary rock. To relate this to rocks, the "dumping" of rocks is done through erosion. Eventually, after lots of time and pressure, the sediments compact together and form layers that eventually form a rock.

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