Sediments are solid particles that settle out of water or air and accumulate on the earth’s surface, either as loose deposits or as sedimentary rocks. The study of sediments is called sedimentology.
Most sediments consist of minerals, but they may also contain organic matter, such as shells, wood, and plant remains. The composition of sediments varies widely from place to place and from time to time.
The formation of sediments begins with the weathering and erosion of rocks at the earth’s surface by water, wind, ice, or gravity. This process breaks the rock into small pieces (sediment). The size of these pieces depends on the type of rock and how it was broken up.
Erosion continues until the sediment is transported to a new location by water (rivers and streams), wind (deserts), ice (glaciers), or gravity (hillslopes).
Once the sediment reaches its new location, it may be deposited in a number of ways:
-Rivers transport sediment in their currents and deposit it when the current slows down near their mouths; this forms deltas.
-Streams flowing over soft bedrock erode wide valleys with steep sides; this is known as valley widening.
– Desert winds blow sand into dunes.
-Ice sheets carry boulders along till they are dumped at the margins; this is known as glacial till deposition.
Valleys may be deepened by any type of erosion but most commonly by rivers through what is called valley deepening . There are three main typesof valleys depending on their shape which are V -shaped , U -shapedand flat bottomed . V -shaped valleys have steep sides formedby rivers cutting through resistant bedrock such as mountainswhile U – shaped valleys have been carved out by glaciersleaving behind a characteristic concave profile . Flat bottomedvalleys occur where rivers flow across very gentle slopesformed by weak bedrock such as shale . As a result , many tributariesjoin together to form large trunk rivers in lowland regionssuch as floodplains . Floodplains are areas adjacentto trunk rivers that experience periodic flooding whenthere is heavy rainfall upstream or when riverbanks collapseinto the river channel due to erosion .