Like the other summits of the Greensand Ridge in the south of Surrey, the rock of which Leith Hill is composed, is primarily the Lower Greensand, overlaid with a harder layer of chert. The greensand was deposited in the early Cretaceous, most likely in a shallow sea with low oxygen levels. Over the subsequent 50 million years, other strata were deposited on top of the Lower Greensand, including Gault clay and the chalk of the North and South Downs.