Sand, gravels and alluvium deposits that overlie the mudstone bedrock occur almost along the entire length of the river, and are an important feature of the middle and lower reaches, with the alluvial river silt producing fertile soils that are used for intensive agriculture in the Trent valley. Beneath the alluvium are widespread deposits of sand and gravel, which also occur as gravel terraces considerably above the height of the current river level. There is thought to be a complex succession of at least six separate gravel terrace systems along the river, deposited when a much larger Trent flowed through the existing valley, and along its ancestral routes through the water gaps at Lincoln and Ancaster.