The Quantock Hills are largely formed by rocks of the Devonian period, which consist of sediments originally laid down under a shallow sea and slowly compressed into solid rock. In the higher north-western areas older Early Devonian rocks known as Hangman Grits (or, more formally, the Hangman Sandstone Formation) predominate and can be seen in the exposed rock at West Quantoxhead quarry, which was worked for road building. The Hangman Grits are described in three divisions: the lowest are the Little Quantock Beds, which are located near Crowcombe, and made up of siltstones and slates. Between Triscombe and West Quantoxhead is a layer of the Triscombe Beds which is around thick and is made up of green sandstone and mudstones. The uppermost division is the Hodders Combe Beds of sandstone and conglomerate and is approximately thick.