The Stones are made of natural boulders of Jurrasic oolitic limestone which forms the main structure of the Cotswold hills and is ultimately derived from an ancient seabed c.160 million years ago. This sedimentary rock consists of tiny spherical grains (ooliths) and other fossilised fragments of sea creatures cemented together by calcium carbonate.
Down the millennia Cotswold limestone has been used extensively for building everything from ancient monuments to churches, houses and stone walls. From the early neolithic period onwards most of the stone used in more elaborate structures and buildings was quarried, but simple megalithic monuments like the Rollright Stones are constructed of surface boulders, in this case found within 500m of the site.
The boulders used to form the Stones are rocks that were originally eroded from the base of the limestone strata (known as Chipping Norton Limestone) by geological action: a faulted rift valley of the Swerford brook to the south and land slipping (possibly related to an enormous glacial lake) to the north. Before being dragged to the locations where the three monuments were built, the boulders had been lying for millennia on the ground surface, exposed to the elements. As a result they are naturally pitted and eroded through the chemical interaction of plants, water and air, together with frost and wind that have etched out weaknesses in the limestone. In some cases holes were created through the stones.
Weathering has also continued since the Stones were erected — probably at a rate of about 0.75-1.0mm per century. A 30mm deep kamenitza (a hollow with an overhanging lip) etched in the top of one of King’s Men entrance stones is typical of limestone exposed to rainwater weathering. The leaning portal stones of the Whispering Knights were probably erected resting against the closing slab between them, but no longer touch it due to dissolution of the limestone. The surfaces of various stones exhibit flaking due to frost damage.
Recently recovered unweathered blocks of the same limestone have been placed in the copse by the King’s Men and on the ridge behind the King Stone for visitors to sit on, climb over and observe fossils visible in the surface – which is not possible with the ancient stones because of their complete covering of lichens and other plant growth.