Geology
The ridge is formed of alternating sandstones and clays known as the Hastings Beds dating from the Lower Cretaceous. Some beds contain iron which was used in the iron industry in the 16th century. A particularly strong sandstone bed is known as Tilgate Stone, but this term has also been used for the whole formation. It was frequently quarried for buildings. A thinly bedded layer is known as Horsham Slate, being quarried to the SE of Horsham, and was used for pavements and roofs. In some examples it has a corrugated surface of ripple marks.
The streams flowing north from the forest are known as brooks and eventually form the River Mole. Those flowing south are called gills and form the River Arun. The latter have cut down exposing bedrock in places. The Forest Ridge is therefore the watershed between the River Thames and the South Coast.
The southern edge of the forest drains into the River Adur, and the Ouse drains the south-eastern corner.
Read more about this topic: St Leonard's Forest