Stone felt that a bluestone monument had earlier stood near the nearby Stonehenge Cursus and been moved to their current site from there. If Mercer's theory is correct then the bluestones may have been transplanted to cement an alliance or display superiority over a conquered enemy although this can only be speculation. An oval-shaped setting of bluestones similar to those at Stonehenge 3iv occurs at Bedd Arthur in the Preseli Hills, but that does not imply a direct cultural link. Some archaeologists have suggested that the igneous bluestones and sedimentary sarsens had some symbolism, of a union between two cultures from different landscapes and therefore from different backgrounds. Recent analysis of contemporary burials found nearby known as the Boscombe Bowmen, has indicated that at least some of the individuals associated with Stonehenge 3 came either from Wales or from some other European area of ancient rocks. Petrological analysis of the stones themselves has verified that some of them have come from the Preseli Hills but that others have come from the north Pembrokeshire coast and possibly the Brecon Beacons. The main source of the bluestones is now identified with the dolerite outcrops around Carn Goedog although work led by Olwen Williams-Thorpe of the Open University has shown that other bluestones came from outcrops up to 10 km away. Dolerite is composed of an intrusive volcanic rock of plagioclase feldspar that is harder than granite. Aubrey Burl and a number of geologists and geomorphologists contend that the bluestones were not transported by human agency at all and were instead brought by glaciers at least part of the way from Wales during the Pleistocene. There is good geological and glaciological evidence that glacier ice did move across Preseli and did reach the Somerset coast. It is uncertain that it reached Salisbury Plain, although a spotted dolerite boulder was found in a long barrow at Heytesbury, which was built long before the stone settings at Stonehenge were installed. One current view is that glacier ice transported the stones as far as Somerset, and that they were transported from there by the builders of Stonehenge. However, in 2015, researchers reported they had confirmed the Preseli Spotted Dolerite stones at Stonehenge came from two Neolithic quarries at Carn Goedog and Craig Rhos-y-felin in the Preseli Hills. Using radiocarbon dating, researchers dated the quarry activities to around 3400 BC for Craig Rhos-y-felin and 3200 BC for Carn Goedog.