Peat is made of partially decomposed Sphagnum mosses and other plants that have accumulated in waterlogged conditions during the last 10,000 years or so. Peat built up during periods of wetter climate and may have been influenced by the woodland clearance of early settlers. It accumulated in valleys and hollows and as patchy spreads on higher ground. There are areas of thin peat on Crosby Ravensworth Fell, Crosby Garrett Fell, Ash Fell and Ravenstonedale Moor. Thicker accumulations are found in the Sunbiggin Tarn and Tarn Moor area where peat is up to 3m thick.