A tor is a large, free-standing residual mass (rock outcrop) that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest. In the South West of England, where the term originated, it is also a word used for the hills themselves particularly the high points of Dartmoor in Devon and Bodmin Moor in Cornwall. The word tor (Cornish tor, Old Welsh twrr, Modern Welsh tr, Scots Gaelic trr), meaning hill, is notable for being one of the very few Celtic loanwords to be borrowed into vernacular English before the modern era such borrowings are mainly words of a geographic or topographical nature. Similar words include crag (from the Welsh word craig, meaning 'rock') and avon (from the Welsh word afon, meaning 'river'). Tors are usually composed of granite or metamorphic rocks, but they can also develop in volcanic rocks and occasionally other hard rocks such as quartzite. They are the result of millions of years of weathering.