Beacon Hill, which rises to nearly 300 metres above sea level, is the most eastern of four upstanding domes, or periclines, forming the ancient core of Mendip. These periclines comprise the oldest rocks in Somerset, deposited by volcanic activity over 420 million years ago, during a time known to geologists as the Primary Era (or Palaeozoic). Stones formed from lava (called andesite) were overlaid with fragments of explosive debris and ash (called tuff).
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A layer of Old Red Sandstone was deposited some 380 million years ago when the climate was sub-tropical desert. The more resistant beds of sandstone, forming the present crest of the scarp, contain rounded pebbles of polished white quartz and are known as conglomerates; the rusty red colour results from the oxidation of iron minerals under arid conditions. Over the following 250 million years the area was covered by sea for extensive periods and a layer of limestone built up. Continued deep burial, compaction and subsequent earth movements caused folds and faults to occur and the Mendip Hills were uplifted about 280 million years ago. Erosion of the hills has since formed a layer of drift over the lower ground and fragments falling from the crest have formed scree at the base of the scarp. Only on Beacon Hill has erosion exposed the underlying andesite rocks. These are quarried for road stone nearby at Stoke St Michael which is the probable site of the volcano core.