This stone is otherwise known as Clach a' Druidh. It's a massive lumpy glacial erratic boulder, over six metres long and broad, which rises four metres out of the ground. Obviously it's the rock of the druids. Slightly less convincing is to link the name with Roderick Hael, a king of Strathclyde. This page from the Geological Conservation Review says it is composed of a basalt, criss-crossed by haematite veins. A quick websearch suggests it's currently (and probably always has been) the haunt of rock climbers. If you can't get to the top, BRAC has some photos of some cupmarks that are up there.
The Clochoderick stone is a particularly fine example of a glacial erratic. Composed of volcanic rock differing from that below the soil on which it rests, but similar to bedrock occurring in the hills a few miles to the north and west, and deposited here during the last ice age around 18,000 years ago.
Legend asserts its use by the druids as a site for dispensation of justice on this rocking stone. Depending on how the stone moved, with the accused in place, innocent or guilty was decreed.
It is also asserted that the stone marks the burial place of Rydderick Hael, monarch of the kingdom of Strathclyde. Considerable research establishes his crushing victory over paganism at Arthuret, near Carlisle, resulting in the conversion of the country to Christianity.