Alluvial and lacustrine calcareous deposits occur throughout. Groundwater occurs in silty to sandy layers of the alluvial sediments. Sedimentary gravels are found in the form of the alluvial Mangas Valley gravels. The lowland consists of alluvial soils. Valleys of large rivers are rich in alluvial soils. The first consists mainly of sand dunes and clayey alluvial soils in the polders. Overlying the bedrock are deposits of glacial sand and gravel, clay, and some alluvial deposits. Examples of continental environments are lagoons, lakes, swamps, floodplains and alluvial fans. It also prefers to grow in siliceous, calcareous, sandy, alluvial and clay soils. These indicator minerals are generally sought in stream sediments in modern alluvial material. From the Pleistocene alternation of maritime and alluvial sediments occur as far west as Piacenza. Many aquifers, such as alluvial aquifers, occur entirely within regolith. Most giant sequoia groves are on granitic-based residual and alluvial soils. The salinity field defines oceans, mangroves or alluvial plains. These plains are formed of old alluvial deposits.