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The Laying of Sedimentary Rocks Over Time


Sedimentary rock forms from bits of rocks deposited over time by wind and/or water. Rocks occur in all sizes. The tiny amounts of mud, pebbles, sand grains, and smaller rock particles can form sedimentary rock. Sediment can also contain the remains of once living things. This may be bones, shells, droppings, leaves or stems. Wind and water carry sediments and deposit them in layers usually in a lake or ocean bottom. The processes that turn sediments into solid rock are compaction and cementation. The processes of compaction and cementation together are known as sedimentation.



Destructive forces are constantly breaking up and wearing away the rocks of the Earth. These forces include: grinding ice, waves, rain, and heat. Erosion occurs when these agents loosen and carry away the fragments of rock or once living things somewhere else.


Deposition Environments: a place where sediments collect.

Eventually, wind or water will drop these fragments. They will accumulate in low valleys or in river bottoms. Deposition is the process by which wind or water sediment settles out in layers. Furthermore, over time, the remains of living things may harden in this sediment and change into fossils. After sediment is deposited the processes of compaction and cementation change the fragments into sedimentary rock.




At first, the fragments of rock lie loosely together. But gradually, over many, many years these thick layers build up. This build up becomes heavy and presses down on the layers beneath it. Compaction is the pressing down of layers forcing the sediments to fit closer together. Over millions of years, this process can squeeze fragments tightly together. The layers often remain visible in the rock and may be used to identify sedimentary rock.



During the process of compaction, the minerals in the rock are dissolving. These dissolved minerals fill in the spaces between sediment particles. Cementation is the process of sediments being glued tightly together. The processes of erosion, deposition, compaction, and cementation may occur over millions of years transforming rock fragments into solid rock.

*** Remember that the processes of compaction and cementation together are called sedimentation.


  1. How does the deposition of rock materials produce layers of sediment?
  2. Over time, how do these layers of sediment become sedimentary rocks?
  3. What force is involved in compacting layers of sediment?
  4. Why are some sedimentary rocks harder than others?

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Updated October 24, 2008 by: Glen Westbroek

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