Launceston. Report from the Geology Section
|Author(s):||Mather. John||Origin:||Section Conference Reports|
The Devonian and Carboniferous rocks of southwest England crop out within six juxtaposed east/west trending basins. The Launceston area lies within the Tavy Basin, close to its border with the Culm Basin to the north, which was the last of these basins to form. The stratigraphical successions within the various basins are structurally complex as a result of early episodic rifting, followed by convergence during the Variscan Orogeny.
The town of Launceston lies on the southern side of the River Kensey, about two kilometres west of its confluence with the River Tamar. About 10km to the southwest looms the granite mass of Bodmin Moor with Dartmoor slightly further away to the east. The land between the two granite masses was mapped under contract by the University of Exeter between 1978 and 1981, and a map published in 1993.
At Launceston, rocks of Upper Devonian and Lower Carboniferous age form an inlier within Upper Carboniferous rocks. Faulting and thrusting mean that most boundaries are tectonic, and the relationships between different formations often obscure. The oldest rocks of the inlier formed approximately 359 to 372 million years ago in the Devonian Period in shallow carbonate seas. Named the Stourscombe Formation and consisting of slatey mudstones and limestones, they have a limited outcrop but are found on the flanks of Windmill Hill. Of comparable age are the mudstones and siltstones of the Liddaton Formation, which are fine grained pelagic deposits formed in open seas. These formations are overlain by black and green mudstones with siltstone laminae and scattered sandstones, again formed in open seas. Evidence recorded in goniatite and trilobite faunas suggests that the conformable Devonian/Carboniferous boundary lies within these sandy mudstones, which are known as the Yeolmbridge Formation and underlie much of the old town.
The Upper Carboniferous rocks formed approximately 318 to 328 million years ago. Known as the Crackington Formation, and some hundreds of metres in thickness, they consist of rhythmically bedded, dark blue-grey mudstones with subordinate grey sandstones and siltstones. The sandstones are turbidites deposited on sub-aqueous slopes in a basinal environment. Locally there are basaltic lavas and pyroclastic deposits formed from explosive eruptions of silica-poor magma. Such igneous rocks form the ridge west of Launceston, an outlier caps Windmill Hill and the castle is built on them. Although no longer visible, distinct pillow structures were recognised by early geological surveyors, indicating subaqueous eruption.
A small Tertiary Basin at Dutson, about 2 km north of Launceston, contains fluvial kaolinitic clays, lignitic clays and clayey sands, which are comparable to those found in the Bovey and Petrockstow Basins in Devon. Their clay mineralogy suggests formation in the Upper Eocene, some 34 to 56 million years ago, during tropical weathering of the underlying Crackington Formation. Dutson was the site of the Launceston Brick Works, which operated from the early nineteenth century until closure in the 1930s. The Crackington Formation also yielded a poor-quality slate from Bangor Cornish Quarry, now the site of the Pennygillam Industrial Estate, on the south side of the A30. The slate did not cope well with the Cornish weather and was suitable for flagstones rather than roofing slate.