Westminster Bank Quarry

Exposed Units: Malverns Complex

Conservation status:  Local Geological Site

Westminster Bank Quarry is a large quarry on the western side of the Malvern Hills. The rock face runs along the eastern side of the site and is approximately 60m in length, and 20m high. The best exposure is found in the middle of the quarry face, the northern and southern parts of the quarry face are obscured by vegetation.

A narrow 2m band of diorite is present in the southernmost part of the quarry face, where it is faulted against granite. The diorites have undergone regional metamorphism, which means they have been subject to high temperatures and pressures in the past. During metamorphism, chemical reactions occur to create new minerals, and the minerals also elongate to produce the schistose fabric. Due to this alteration the diorite has a rotted appearance, with the chlorite and clay minerals developed from the breakdown of the minerals hornblende feldspar respectively.

Faulted against the diorite is coarse grained granite comprised of the minerals orthoclase feldspar and quartz; this is the dominant rock type at the site. These coarse grained granites vary throughout the 60m exposure; the northernmost granites are strongly foliated, with an 8m section where the granite has a gneissic appearance. In the southern half of the quarry, normal faulting introduces a complex sequence of granites and diorites, both of which have been extensively altered.

Westminster Bank Quarry affords views out across Herefordshire to Wales enabling people to view the changes in landscape and therefore the changes in geology. The undulating hills made of Silurian shales and limestones level out onto the Herefordshire plain, made of Devonian sandstones and mudstones. The Woolhope Dome rises out of this plain and forms a prominent feature in the landscape. On days when the weather is clear the Welsh Mountains can be seen.

This site is part of the Community Earth Heritage Champions Project.

Terminology

Fault – A line of weakness within the Earth’s crust along which movement and displacement occurs.

Normal Fault – Movement along a line of weakness in the Earth’s crust where the handing wall is displaced downwards relative to the footwall.

Schistose Fabric – A foliation produced by deformation in which tabular and platy minerals, coarse enough to be visible to the unaided eye, have a preferred orientation.

Gneissic appearance – A foliation of compositional layering, formed during deformation and metamorphism.

Dyke – A sheet like, near vertical minor intrusion.

Sheared – Where a rock is subject to directional stress (i.e. pressure with a directional element), minerals in the rock to realign according to the direction in which the pressure is directed.

Relative dating – Dating of rock units in relationship to each other e.g. rock A is younger than B but older than rock C.

Photos

Coarse granite in the centre of the quarry (Facing NE).

Foliated granite with fault (Facing NE).

Close up of coarse granite from Westminster quarry.

References

Barclay W.J. et al. (1997) Geology of the county around Worcester. British Geological Society Memoir. London.

British Geological Survey (1993) Geology of the country around Worcester. Sheet 199: 1:50,000

Bullard D.W. (1989) Malvern Hills; A student’s guide to the geology of the Malverns. Nature Conservancy Council. Peterborough.

Butcher N.E. (1962) The Tectonic Structure of the Malvern Hills. Proc. Geol. Soc. 73 pp.103-123.

Falcon N.L (1947) Major Clues in the Tectonic History of the Malverns.

Fitch F.J. (1966) Isotopic age determinations on rocks from Wales and the Welsh Borderland. pp. 22-45 in Pre-Cambrian and Lower Palaeozoic rocks of Wales. Wood, A (Editor). University of Wales Press.

Kellaway G.A. and Hancock P.L. (1983) Structure of the Bristol District, Forest of Dean and the Malvern Fault Zone. in The Variscan Fold Belt in the British Isles. Hancock P.L. (Editor). Adam Hilger LTD. Bristol.

Lambert R. St J. and Holland J.G. (1971) The Petrography and Chemistry of the Igneous Complex of the Malvern Hills, England. Proc. Geol. Ass. 82. pp. 323-352.

Penn J.S.W. and French J. (1971) No 4: The Malvern Hills. Geologists Association Guides.

Pharoah T.C. et al. (1987) Geochemical Evidence of the Tectonic Setting of Late Proterozoic Volcanic Suites in central England. Geol. Soc. Special Publication No. 33. pp. 541-552.

Pharoah T.C and Gibbons W. (1994) Chapter 10: Precambrian rocks in England and Wales south of the Menai Strait Fault System. from Gibbons W. and Harris A.L: A revised correlation of Precambrian rocks in the British Isles. Geol. Soc. Special Report No. 22.

Phipps C.B. and Reeve F.A.E. (1967) Stratigraphy and Geological History of Malvern, Abberley and Ledbury. Geological Journal. 5 (2).

Timins Rev J.H. (1867). On the Chemical Geology of the Malvern Hills. Proc. Geol. Soc.

Thorpe R.S. (1972) Possible subduction zone origin for two calc-alkaline plutonic complexes from Southern Britain. Bulletin of the Geological Society of America. 83 pp. 115-120.

Thorpe R.S. (1987) Psuedotachylite from a Precambrian shear zone in the Malvern Hills. Proc. Geol. Ass. 98 (3) pp. 205-210.

Tucker R.D. and Pharoah T.C. (1991) U-Pb zircon ages for Late Precambrian igneous rocks in Southern Britain. Journal of the Geological Society. 148 pp. 435-443.

Woodcock, N and Strachan, R. 2000.Geological History of Britain and Ireland. Blackwell Publishing.

March 2011