The thick, main or scaur limestone (mountain limestone) of the centre and south of England, Wales and Carboniferous Ireland, which splits up in the Yorkshire dales C Limestone (Yoredale group) into a succession of stout limestone Li Series.
The castle was built during the 12th century by William le Gros, earl of Albemarle, who chose the site on the top of a steep cliff now called the "Scaur."
The chief are Aysgarth Force, on the main stream, Mill Gill Force on a tributary near Askrigg, and Hardraw Scaur beyond Hawes, the finest of all, which shoots forth over a projecting ledge of limestone so as to leave a clear passage behind it.