Geological Report

Geological Notes on the Beacon Hill inlier, Shepton Mallet

Beacon Hill lies on the eastern edge of four upstanding domes forming the ancient core of Mendip. These so-called periclines now give rise to prominent and distinctive high ground above the general surface of Mendip’s limestone plateau. Their summits are around 300 metres above sea level. The periclines comprise of the oldest rocks in Somerset, deposited over 400 million years ago (mya) during a time known to geologists as the Primary Era (or Palaeozoic). Although completely buried by younger rocks during the ensuing Secondary Era (or Mesozoic), they were re-exposed later by erosion and severe weathering towards the end of Tertiary and Quaternary times (or Cenozoic) in particular. Mendip experienced arctic-like conditions for lengthy phases during the so-called Ice Ages over the past 1 million years. The present landscape was thus shaped as the sea lowered to its present day level.

The Beacon Hill Wood Reserve contains much useful evidence of the above ‘geological story’, especially along the former Roman Fosse Way which climbs over Mendip from Shepton Mallet towards Bath. This trackway winds up the steep south facing escarpment to the top of the ridge within the heart of the wood. The well-trodden route at the foot of the scarp face traverses weathered volcanic rocks. These formed on the slopes of a nearby island arc volcano that was active 420 mya during Silurian times. Nowadays, the rubble underfoot reveals stones of lava (called Andesite) with fragments of explosive debris and ash thrown further from the erupting crater (called tuff). The former rock is extensively quarried for roadstone at nearby Moon’s Hill Quarry, Stoke St. Michael.

The prominent west-east ridge along the length of the wood adjacent to the Old Frome Road (also a former Roman Road), comprises of resistant Old Red Sandstone deposited some 350 mya during Devonian times. Thick beds of these consolidated desert-derived quartz sands are exposed in small bluffs, cliffs and old quarries along the south facing crest of the scarp. The rusty red colour of these rocks results from oxidation of iron minerals under arid conditions, as in today’s sub-tropical desert regions. The most resistant beds of sandstone forming the prominent crest of the scarp exhibit many rounded pebbles of polished white quartz typical of flash flood events. These are known as conglomerates.

Deep burial, compaction and subsequent earth movements have caused cracks to open up in the Old Red Sandstone, called bedding and joint planes. The former show that these beds were tilted northwards locally (some 28 degrees from the horizontal) when the Mendip Hills were uplifted about 280 mya. Many vertical joints are also evident in the ORS outcrops along the cliffs. Surface water penetrating them has eventually created weaknesses for erosion to occur, especially during rainstorms and severe frosts. Angular fragments of ORS fallen from the cliffs over many years have produced scree slopes at the foot of the escarpment. Embayments in the eroded scarp face east of the Fosse Way also indicate the existence of local fault lines cutting through the compressed core of the Beacon Hill pericline. These faults give rise to small springs along the southern boundary of the reserve. In general, however, the ORS is relatively impermeable and the thick soils developed throughout the woodland are acid, peaty and gleyed (where saturated).

Jim Hanwell