Tavistock. Report from the Geology Section

Geology has had a huge influence on the appearance of present day Tavistock, affecting both its social history and buildings. This is so important that the town is in part of the UNESCO Cornwall and West Devon Mining Landscape World Heritage Site – the ‘Tamar Valley and Tavistock Mining Area’.

Tavistock was one of the Stannary towns where from the 12th century tin was weighed, stamped and assessed for duty. After a decline in tin mining, industrial scale mining followed in the sixteenth to twentieth centuries and the region dominated world output of copper and arsenic. The major mines around Tavistock were operated by Devon Great Consuls, with activity at Mary Tavy and Peter Tavy. The need to export the metals mined resulted in the establishment of small quays such as that at Morwellham on the Tamar.

The Bedford family in particular made a great deal of money from this and in the nineteenth century the seventh Duke of Bedford remodelled the town centre using royalties from the mines. However, it is not just the grand public buildings that are important but also the housing built for the workers that represented a considerable shift in attitude from employers. Part of the inscription for the UNESCO site states that “it was the technological, social and economic contribution made by the mining industry that was crucial to the development of modern industrial and capitalist society”.

Tavistock has some distinctive building stones, including the local green volcanic Hurdwick Stone and granite from Pew Tor. The abbey that was demolished after the Dissolution of Monasteries was made of Hurdwick Stone and much of this stone was reused in newer buildings. The Hurdwick Stone is from the Milton Abbot Formation that consists of basaltic lavas and volcaniclastic deposits (tuff/hyaloclastite). It was quarried locally taking its name from Hurdwick Farm. Its colour is distinctive and it has a rough texture in many buildings due to holes in it. Some of these holes are amygdales or vesicles but others are the holes left by bits of devitrified volcanic glass that have fallen out. These are distinguished by the fact that they are not ellipsoidal in shape (derived from roughly spherical gas holes) but have irregular outlines because they started out as angular shards. Unfortunately the stone doesn’t always weather well (probably due to the rough texture) but it was extensively used in the Duke of Bedford’s rebuilding of the town centre.

There is a County Geological Site at the Tavistock railway cutting (SX 4722 7413 – SX 4788 7448) where the bedrock greenish Devonian slates and fine grained volcanic ash deposits and lavas that underlie Tavistock are exposed.

Jenny Bennett and Mike Barr