Beckford Gravel Pit

Exposed Units: Post-Anglian Deposits (Wasperton Member)

Conservation Status: Site of Special Scientific Interest

The locality contains a significant cliff exposing interbedded sands and gravels that were deposited by and around the ancient River Avon. Gravel beds are 10-30 cm in thickness and sand beds are generally thinner but also form wedges that may be up to 20 cm thick. The gravel clasts have been derived from material eroded by the river and are dominantly limestone, although occasional grains of a dark, nodular rock of uncertain affinity are present. Sand grains are generally very rounded and spherical, indicating that the sand may also have been wind-blown before it formed part of the Wasperton Member.

The transitions from gravel to sand layers are sharp, indicating erosive surfaces, or representing sudden changes in the carrying capacity of the river; a fast-flowing, more powerful river (for example, a river in flood), is able to transport larger particles than a sluggish river. In the lower part of the section, there are possible slumping, buckling and frost heave features, which indicate deposition in a cold climate that is significantly influenced by repeated freeze-thaw events. In the upper part of the section, the gravel beds become thinner. In the topmost metre of the section, very large clasts indicate periods of much higher energy transport, possibly as a result of the conditions at the end of the Last Ice Age (Devensian).

The characteristics of the Wasperton Member at this locality indicate that the overall climate at the time of its deposition was cold (but not covered in ice) and arid, suggested by the presence of freeze-thaw structures and wind-blown sand respectively. The sand is not local, with the nearest possible source being the Triassic sandstones of northwest Worcestershire. The limestone clasts which make up the gravels are likely to have been sourced from Bredon Hill, where there are extensive outcrops of Jurassic limestone.

Terminology
Clast – A rock fragment that has been incorporated into a rock from another rock.

Carrying capacity – The ability of a river to transport a sediment load.

Freeze–thaw structures – Weathering features in rocks caused by the repeated freezing and thawing of water contained in fractures and pore spaces.

Deglaciation – The process of glacier retreat and melting at the end of a glacial period. Opp. glaciation.

Photos

General view of the cliff face exposure at Beckford Gravel Pit showing horizontally-layered sand and gravel beds in the Post-Anglian age Wasperton Member.

Discontinuous bands ofcoarse gravel surrounded by finer sand.

Photograph showing the variable and repetitive nature of river deposits. To the right of the photo is a wedge of very coarse gravel that pinches out towards the left of the image. This is overlain by a wedge of sand that pinches out towards the right of the photo, which has coarse-grained material at the bottom but becomes finer moving up. These are both overlain by continuous bands of gravel and sand at the top of the image.

Very coarse, angular clast being weathered out of the cliff face. The finer-grained sand and gravel is more easily eroded by wind and weather, which slowly picks out the larger grains.

Slumping in a sand wedge of the Wasperton member. Sediment deposition on a slope in a river channel caused the slope to become unstable. A lens of material slumped down onto underlying sandy material. The direction of slumping is moving towards the camera. The slump structure is overlain by a band of gravel that would have scraped away the top layers of the slump when it was deposited, so forming an erosive boundary.

References
Briggs, D.J., Coope, J.R. and Gilbertson, D.D., 1975, ‘Late Pleistocene terrace deposits at Beckford, Worcestershire, England’, Geological Journal, 10, pp. 1-17.

Brown, A.G., 1983, ‘Floodplain deposits and accelerated sedimentation in the lower Severn Basin Article’. In: Gregory, K.J. (eds.), Background to palaeohydrology, Wiley, Chichester, pp. 375-397.

Dawson, M.R., 1986, ‘Late Devensian fluvial environments of the Lower Severn Basin, UK’, PhD Thesis, University of Leicester, Leicester.

Gregory, K.J., 1997, ‘Beckford, Hereford & Worcester’. In: Gregory, K.J. (eds.), Fluvial geomorphology of Great Britain, Geological Conservation Review Series, 13 JNCC, Peterborough, pp. 347.

March 2011