* Erosion and Transport: The process of weathering breaks down existing rocks into smaller fragments, called clasts. These clasts are then transported by wind, ice, or water. Water, especially flowing water, is a much more efficient transporter of sediment than wind or ice. This is because water has a higher density and viscosity, allowing it to carry larger and heavier clasts.

* Deposition: As water slows down, whether due to entering a lake or ocean, encountering a shallower area, or encountering a change in flow rate, it loses energy. This causes the transported clasts to settle out and accumulate on the bottom. This accumulation of sediment is called deposition.

* Lithification: Over time, the deposited sediment layers are buried under more sediment. The weight of overlying layers compresses the sediment, squeezing out water and air. Dissolved minerals in the water then precipitate and act as cement, binding the clasts together. This process of turning sediment into solid rock is called lithification.

Exceptions:

While most clastic sedimentary rocks form underwater, there are some exceptions:

* Wind-blown Sand: Deserts and coastal areas can accumulate significant amounts of sand transported by wind, forming eolian deposits. These deposits can later become lithified into sandstone.

* Glacial Till: Glaciers can transport and deposit a wide range of sediment, including boulders, gravel, sand, and silt. This unsorted deposit, called glacial till, can also form sedimentary rock.

* Fluvial Deposits: Rivers can transport and deposit sediment in floodplains and riverbeds, forming fluvial deposits. These deposits can also become lithified into sedimentary rock.

In Summary:

While there are exceptions, the vast majority of clastic sedimentary rocks form underwater because:

* Water is a more efficient transporter of sediment than wind or ice.

* Water slows down and deposits sediment in lakes, oceans, and other aquatic environments.

* Sediment is buried and lithified under the weight of overlying sediment.