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The Brassington Formation of the southern Pennines comprises a succession of sands, gravels and clays, the youngest beds of which contain Lower Pliocene fossil plants. These strata are generally considered to have foundered into the Carboniferous Limestone through solution subsidence mechanisms. The subsidencc cavities also contain detached marginal blocks of shale, which locally contain Namurian goniatites.
Examination of the shale blocks from a number of widely separated localities has revealed that all contain Namurian spore assemblages, datable in some cases as E2 (Arnsbergian) age. The contact between the shale blocks and oldest sands of the Brassington Formation is unconformable and, characteristically, the latter are seen to rest on zones of lilac-stained weathered shale, representing a pre-subsidence land surface.
Evidence is presented which indicates that the Brassington Formation strata are conformable and that the succession represents a single cycle of sedimentation. Because of this conformable relationship it is inferred that this land surface is also of Lower Pliocenc (or Upper Miocene) age.
A tacheometric survey of a well-exposed solution subsidence complex at the Bees Nest Pit, Brassington, has enabled structure contour maps to be made of key horizons in the succession. Laboratory tests have been conducted which have attempted to duplicate this structure and so indicate the probable subsidence mcchanisms. Results suggest that subsidence was gradual, with block-by-block removal of the host limestone, and took place in deep, steep-sided pipe-like forms.
The depth of subsidence, and hence the elevation of the land surface prior to the deposition of the Brassington Formation, has been determined by estimating the thickness of limestone and Namurian shale which, as suggested by previous research work in the southern Pennines, was once present over the sites of the present subsidence masses. Subsidence can be shown to be of the order of 150–250 m, which implies that the Lower Pliocene land surface of the southern Pennines lay at about 450m O.D.
Based on the evidence of the contemporary sea level, the presence of the Lower Pleistocene deposits of sE England, and the sub-Neogene surface of the southern Pennines, a graph has been constructed which suggests broadly that Upland Britain has undergone a regular uplift since the main Alpine episode at a rate of about 1 metre in 15,250 years. In this light, the possible age of other well known planation surfaces is discussed.
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