Exposed Units: Llanelly Formation
Conservation Status: Local Geological Site
The Llanelly Formation at this site dips at about 16° south east and displays differing colours and textures of sandstones, nodular limestones and clay bands. The units here are variably massive or thinly bedded, with a tendency for thinner beds to be sandy and yellow in colour. A layer in the middle part of the sequence is very finely crystalline and contains voids in places where material has weathered out. These textures are markedly different from the surrounding rock layers and it is likely that this layer represents the remains of algal mats that developed in a shallow marine setting.
At the top of the exposed sequence, the limestones are nodular and are set within a blue/green clay matrix of striking colours. The formation of these nodular beds may be due to dewatering, soft sediment deformity, or a diagenetic effect. Above the nodular bed, the beds become laminated, and have pronounced current ripple marks in profile, and obviously represent a change in conditions.
Terminology
Diagenesis (sic diagenetic) – A sequence of chemical reactions and processes that occur during the transition from sediment to sedimentary rock.
Photos
General view of the quarry face at Lord’s Wood Quarry.
References
Welch, F.B.A. and Trotter, F.M., 1961, Geology of the country around Monmouth and Chepstow, Memoir of the British Geological Survey, HMSO London
Coones, P., 1991, ‘A review of the stratigraphy and structure of the Lower Palaeozoic rocks in the Forest of Dean’, Proceedings of the Geologists’ Association, vol. 102, 1, pp. 1-24.
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