Sedimentary rocks are formed from broken pieces of rocks. These broken pieces of rock are called sediments. The word "Sedimentary" comes from the root word "Sediment".

Sedimentary rocks are usually formed in water. Streams and rivers carry sediments in their current. When the current slows around a bend or the river empties into a lake, or ocean, or another river the sediments fall out because of gravity. The larger sediments fall out first and the lightest sediments fall out last.

The diagram above shows layers of sediment that were laid down in a lake. In the spring the lake receives an influx of water from the mountain snow melt. This snow melt carries with it a large amount of sediment that becomes suspended in the lake water. As the sediment settles out during the summer and especially in the winter, if the lake becomes frozen over, the sediments come to rest on the bottom. The heaviest and largest particles settle out first and the lightest sediments such as silts and clays settle out last. The number 1 shows sediment that would have been laid down during 1994, number 2 in 1995, and number 3 would have been laid down in 1996. The gray area above the 3 would be the latest layer being laid down at the present time. This laying down of rock-forming material by a natural agent is called deposition. Natural agents of deposition are water, ice, gravity, and wind.

Sediment is deposited in flat, horizontal layers with the oldest layers on the bottom and the younger layers laying on and over the older layers. Geologists use this knowledge to read layers of sedimentary rock like the pages in a book. They can date layers by the fossils that are found in them. If a layer has a fossil in it that is known to be 50 million years old the layer itself must be at least 50 million years old and the layers below it have to be older than 50 million years.

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