In contrast to the over 70 stone circles known from Dartmoor, there are only two known from Exmoor: Porlock Stone Circle and Withypool Stone Circle. The contrast between the number of rings on the two moors may be because Dartmoor has abundant natural granite while Exmoor has none, instead having Devonian slates and Hangman Grits, both of which easily break up into small slabs, resulting in a general shortage of big stones on Exmoor. The two Exmoor stones are made from smaller stones, which archaeologists have termed miniliths. However, the presence of other Exmoor constructions, such as the clapper bridge at Tarr Steps and the three-metre Long Stone at Challacombe, which do use locally sourced large megaliths, suggests that larger stones would have been available had the sites' builders desired, and that the use of miniliths was perhaps deliberate.