Sarsen Stones and Erratics of the Wessex Coast; Geology of the Wessex Coast Field Guides: "Sarsen stones are blocks of quartzite, often found above the Chalk, and most famous for their use at the major stones of Stonehenge (in addition to the Bluestones - dolerite). This webpage is concerned with those on the Wessex coast and adjacent area, with some inland examples also included.
Sarsens are very abundant in the Solent area, particularly from the coast at Chilling, near Fareham, southeast to Selsey Bill. They are particularly features of the low-level Pleistocene gravel terraces and the low-level raised beach of the Selsey peninsula. The Solent type of sarsens are not very large, often about a metre in length and with a thickness of about 60cm. They are usually joint-bounded and often with moulds of tree roots and small roots of shrubs. They mostly consist of medium sand of quartz, cemented by quartz.
The source area for the sarsen stones of the Solent has not been determined. Their common occurrence in southern England above the Chalk indicates a Tertiary origin."