Southcote, Berkshire of chalk, with geological surveys also finding flint samples. A map produced by the Geological Survey of Great Britain in 1860 identifies the area as being predominantly Plastic Clay (now known as the Lambeth Group) and chalk north of the river, with flint and gravel samples typical of the Bagshot Formation south of the Kennet. More specifically, the 2000 survey showed different types of gravel, including Winter Hill (variably clayey and sandy), Lynch Hill (sand and gravel with lenses of silt, clay or peat), and head (a polymict deposit usually formed by periglacial solifluction or gelifluction). The survey also identified
Silt out which is also likely to disturb silt where it makes contact, and the underwater work may also disturb silt. For these reasons surface supplied divers must often operate in very poor visibility. Remotely operated underwater vehicles and submersibles manoeuver by using thrusters, and the water blast from thrusters has a similar effect to the wash from fins. Silt outs are dangerous situations for scuba divers, particularly in enclosed spaces or when there is no direct access to the surface (an overhead environment). Training courses in overhead environment diving, such as wreck or cave diving teaches various methods to cope with