Sediment and Sedimentation

Generally, erosive agents remove sediments from the site of weathering in one of three ways: impact of the agent, abrasion (both types of mechanical erosion, or corrasion), or corrosion (chemical erosion). The mere impact of wind, water, and ice erodes sediments; for example, flowing water exerts a force on sediments causing them to be swept away. The eroded sediments may already be loose, or they may be torn away from the rock surface by the force of the water. If the flow is strong enough, clay, silt, sand, and even gravel, can be eroded in this way.

Abrasion is the second mechanism of sediment erosion. Abrasion is simply the removal of one Earth material by the impact of another. Rock hounds smooth stones by "tumbling" them in a container with very hard sand or silt particles known as abrasives. When you use sand paper to smooth a wood surface, you are using the abrasive qualities of the sand embedded in the paper to erode the wood. In nature, when water (or wind or ice) flows over a rocky surface (for example, a stream bed), sedimentary particles that are being transported by the flow strike the surface, and occasionally knock particles loose. Keep in mind that while the bedrock surface is abraded and pieces are knocked loose, the particles in transport are also abraded, becoming rounder and smoother with time.

Corrosion, or chemical erosion, the third erosional mechanism, is the dissolution of rock or sediment by the agent of transport. Wind is not capable of corrosion, and corrosion by ice is a much slower process than by liquid water. Corrosion in streams slowly dissolves the bedrock or sediments, producing mineral solutions (minerals dissolved in water) and aiding in the production of clastic sediments by weakening rock matrix.