Exposed rock units: Malverns Complex, Malvern Quartzite, Hollybush Sandstone
Conservation Status: Local Geological Site
This quarry exposes a wide range of intensely faulted Precambrian and Cambrian rocks, including distinctive blue-grey very fine grained schist that is defined by chlorite. In the northern part of the quarry, the boundary between the Malvern Complex granites and the Malvern Quartzite is exposed. Along the western edge of the quarry, the Hollybush Sandstone is in contact with the Malvern Complex.
The Precambrian Malvern Complex at Whiteleaved Oak Quarry consists of intensely sheared granite in the form of bands of chlorite schist which wraps around eyes of coarser-grained granite pegmatite. The formation of the chlorite schist may be the result of shearing along the boundary with the Hollybush Sandstone.
The Cambrian Malvern Quartzite is a quartz-rich basal conglomerate that directly overlies the Malverns Complex granite. The conglomerate here is matrix-supported and contains clasts of quartz, minor feldspar and fragments of the Malverns Complex that may measure up to 15mm in diameter.
The Cambrian Hollybush Sandstone here is more impure than the Malvern Quartzite, containing higher proportions of feldspar and clay minerals. Brachiopod and trilobite fossils have been recorded in this unit. The Hollybush Sandstone in the north part of the quarry forms planar beds that have been subject to bending and minor fracturing. This deformation has been attributed to minor faulting in the area, which is also the likely cause of schist formation in the Malverns Complex.
The boundary between the three units in this quarry represents an onlap unconformity, which resulted from a sea level rise during the Cambrian. The Malvern Quartzite and Hollybush Sandstone were deposited on top of Malverns Complex, which were exposed at the surface or on the sea floor. The unconformity and the presence of Malvern Complex pebbles in the conglomerates proved in part that the Malvern Complex rocks were older than the Cambrian sequence. Until then, it was thought that the Malvern rocks had intruded into the Cambrian sequence and so were younger.
Terminology
Schist – A rock fabric formed by deformation that is planar in geometry and defined by platy (sheet-like) minerals, such as biotite, muscovite and chlorite.
Unconformity – A surface between two rock units that represents missing time. An unconformable surface, which is typically an erosive surface, indicates that the deposition of rock units on either side was not continuous.
Photos
References
Bennett, A.G., 1942, ‘The Geology of Malvernia’, Transactions of the Malvern Naturalists’ Field Club, vol. 2, pp.73.
Bullard, D.W., 1989, Malvern Hills – A student’s guide to the geology of the Malverns, Joint Nature Conservancy Council, Peterborough.
Groom, T.T., 1899, ‘The geological structure of the southern Malvern Hills and of the adjacent district to the west’, Journal of the Geological Society, vol. 55, 1, pp. 129-169.
Holl, H.B., 1865, ‘On the geological structure of the Malvern Hills and adjacent districts’, Journal of the Geological Society, vol. 21, 1, pp. 72-102.
Metcalfe, H.F., 1913, ‘Cambrian rocks of the southern Malverns’, Transactions of Malvern Naturalists Field Club, for 1909-1915, pp. 75-84.
Penn, J.S.W., and French, J., 1971, The Malvern Hills, Geologists’ Association Guides, Geologists’ Association
Roberts, G.E., 1860. The rocks of Worcestershire: their mineral character and fossil contents, Joseph Masters, London, pp. 247.
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