These pebbles were deposited in a stream that once flowed over this area in Indiana, USA thousands of years ago. Now they are part of a clastic sedimentary rock called conglomerate.
Click on image for full size
Courtesy of Bruce Molnia, Terra Photographics
Step 3: Deposition (Sediments Settling Down!)
When water or wind loses energy and slows down, sediment can no longer be carried in it. The particles of sediment fall through the water or air and form a blanket of sediment on the bottom of a river, a lake, ocean, or on the surface of the land. This process is called deposition.
Settling out of the wind or water depends on the size of the clast. Larger clasts with high mass will settle more quickly than smaller clasts with less mass. Conversely, it takes more energy to move larger clasts through the water or air (a process called entrainment).
How does deposition happen?
Try this and find out!
- Add a handful or two of sediment to a jar that includes particles of different
sizes such as pebbles, sand and mud.
- Fill the jar with water and seal the lid on top.
- Shake the jar and watch the pebbles and dirt become entrained in the water.
- Let the jar sit still for a few minutes. What has been deposited on the
bottom of the jar? Are any of the small particles still in the water? (The
heaviest items fall out of the water more quickly than the small ones once
you stop shaking the jar.)
Shop Windows to the Universe Science Store!
The Fall 2010 issue of The Earth Scientist
, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!
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