The building stones of Wales are many and varied reflecting the diverse geology of the country. Some of the earliest known use of natural stone for building purposes was the sourcing of Ordovician dolerite in the Preseli Hills for the 'bluestone' lintels of Stonehenge. Other early use was in the construction of dolmens, burial cairns and stone circles in the late Stone Age and Bronze Age. The tradition of building in stone was continued into Iron Age with the establishment of such hill forts as those at Tre'r Ceiri in North Wales and Garn Goch in the south.