Poldark Mine designated a Regionally Important Geological Site

The Geology of the Poldark Mine and its surrounding area.

For Advanced Students

N. G. LeBOUTILLIER BSc., PhD., MCSM., EurGeol., CGeol., FGS.

Introduction

Poldark Mine [SW682315] is situated adjacent to the hamlet of Trenear, 600 metres northeast of the village of Wendron and some 13 km (8 miles) south of Camborne in southwest Cornwall. It lies within the southwestern quadrant of the Carnmenellis Granite (see Figure 1), which is emplaced into metasediments (slates of the Mylor Slate Formation) of Upper Devonian age.

Figure 1. The geology of southwest Cornwall (after BGS 1:250,000 sheet 50N 06W, Lands End).

Southwest Cornwall is an area of physical contrasts (see Figure 2). The granite moorlands form the high ground, reaching a maximum height of 252 m above O.D. on Carnmenellis and Land’s End (at Watch Croft, west of St Ives), with Carn Brea and Carn Marth (satellites of the Carnmenellis Granite) reaching 225 and 235 m above O.D. respectively. Godolphin Hill (in the Tregonning-Godolphin Granite) reaches 194 m above O.D. Most of the granite moorland forms open country with rounded summits. In the west across the Land’s End Granite, a dissected and degraded wave-cut, Pliocene, plateau marks the northern edge of the granite, forming a plain at about 130 m above O.D., behind this a range of rounded granite hills form a prominent ridge (at about 200 m above O.D.) running west to St Just. On the coast the granite forms cliffs commonly 50 to 90 metres high.

Figure.2. The physiography of southwest Cornwall.

The influence of prominent joint sets has enabled the weathering of the granite into large cuboidal blocks, giving rise to the spectacular ‘castellated’ appearance of the cliffs at locations such as Land’s End and Porth Loe (see Plate 1). The erosion along major joint sets, faults and mineral lodes has given rise to the formation of steep, narrow inlets all around the coast known as ‘zawns’.

Away from the moorlands the land falls away from the planated northern coastal strip (at 80-90 m above O.D.) with its prominent cliffs, to the gentle rolling lowland of the south coast (around the Carrick Roads in the Falmouth district, cliff heights reach an average of 10 m above O.D.). Large bays ( Mounts Bay and Hayle Bay) lie behind the Land’s End Granite, and to the east the drainage basin of the Truro, Tresillian and Fal rivers forms a deeply dissected area (a ria system) of steep-sided wooded valleys and open farmland on the floodplain.

The main industries of the area are agriculture and tourism. Much of the upland areas are used for pastureland and the growing of cereal animal feed for the dairy industry, the lowlands area given over to cereal and vegetable production with seasonal flower and fruit farming. The extensive beaches, landscape and clement climate are major draws for tourists and much of the areas workforce are engaged in the tourism and support industries.

The old staple industries of the region, fishing and mining, are now in decline or extinct. The fishing fleet, based primarily at Newlyn, is much reduced and faces an uncertain future in the face of foreign competition and European legislation. The last mine, South Crofty, closed in 1998, bringing to an end some 4,000 years of mining in the region. At its peak in the mid 19 th century some 50,000 people were directly employed in the industry. Today many mining remains are being preserved for posterity as part of Cornwall’s mining heritage (see Plate 2) and as tourist attractions.