The main rocks and minerals to be found in the valley are green slate and granite. There has been some record of opal being found too though it has never been mined. Much of the local stone has been used since pre-history as the raw material for field boundaries using a local technique known as dry stone walling. Just below the reservoir is a Geological Conservation Review site known as Jumb Quarry. The site is significant due to its volcanic rock which displays "bird's-eye tuffs", which contain lapilli (fragments of lava that erupted from a volcanic centre and probably fell into standing bodies of water). The lapilli are believed to have been spherical originally, but were squashed into elliptical shapes as a result of intense pressure during the formation of the Caledonian Mountains.