Geology
During much of the most recent ice ages, from 1.8 million years ago, the sea level in the Severn Estuary was some 50 m (160 ft) below the current level and Flat Holm was joined to the Somerset coast as an extremity of the Mendip Hills. Sometime since the start of the Mesolithic period, 15,000 years ago, the ice sheets retreated, and the flat plains surrounding the river estuary flooded; the hilltops of Mendip Hills became the islands of Flat Holm and Steep Holm.
The Carboniferous Limestone of which Flat Holm and neighbouring Steep Holm are composed forms a part of a wider Mesozoic basin extending from the Bristol and Mendip area westwards beneath the mouth of the Severn to South Wales, outcropping to the west at Sully Island and Barry, Wales. Part of the island is designated a Geological Conservation Review (GCR) Site and is a recognised Site of Special Scientific Interest (SSSI). The GCR and SSSI interest lies along the south-western shoreline from the north west point to Lighthouse Point where a wave cut portion of the limestone displays large fossil "ripple marks".
There are argentiferous (i.e. silver-bearing) galena deposits on Flat Holm; the pits and mounds visible on the surface of the island are a result of trial borings. A dispute over lead mining rights in the 1780s ended with John Stuart, Lord Mount Stuart making an official complaint that the lighthouse keeper was using the coals intended for the lighthouse for processing lead. Mining for lead was not profitable, however, and the works were abandoned. Red marl from the Triassic Period fills joints in the Carboniferous Limestone showing evidence of karstic (cave forming) processes during this period. Caves on the western and north-eastern sides of the island were used during the years of smuggling.
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