What is the most important reason that sediments at beaches are usually rounded and smooth?
Water and wave erosion
http://geography.about.com/od/physicalgeography/a/erosion.htm
The most important reason that sediments at beaches are usually rounded and smooth is due to the constant motion and action of waves.
To understand why sediments become rounded and smooth, let's consider the process involved. At the shoreline, waves continuously crash onto the beach, carrying and transporting sediments such as sand, pebbles, and shells. As the waves approach the shore, their energy increases, creating powerful surges of water that move the sediments along.
During this process, the sediments are subjected to various forces, such as abrasion and attrition. Abrasion occurs when the sediments rub against each other or against hard surfaces, wearing down their rough and angular edges. As a result, the sediments gradually become smoother. Attrition, on the other hand, refers to the breaking down of sediments into smaller fragments as they collide with each other or with other hard objects, further contributing to their rounded shape.
Additionally, the continual action of waves causes the sediments to roll and tumble on the beach, constantly being moved and agitated. This movement helps to polish the sediments and remove any sharp edges, resulting in the rounded and smooth appearance commonly seen on beaches.
In conclusion, the continuous motion and action of waves, along with the forces of abrasion and attrition, play a vital role in shaping and smoothing the sediments found on beaches.