Einar G. Poole writes: Due to mid-Aptian erosion, the significance of the early Aptian unconformity discussed by Ruffel cannot be assessed within the Abingdon-Swindon districts (BGS sheets 252, 253) along the northern margin of the Wessex Basin. However both the mid-Aptian unconformity, at the base of the Lower Greensand (nutfieldiensis Zone), and the early Albian (basalt Gault) unconformity are well developed.

W. J. Arkell (1939,, 1944) was the first to recognize the widespread regional effects of the earth movements which folded and faulted the earlier Jurassic and Lower Cretaceous deposits beneath these unconformities in this part of the Wessex Basin. His views have been fully supported by the six-inch survey of the Abingdon and Swindon districts and the detailed exploration programmes carried out to investigate the relationship and extent of the fuller's earth discovered in the Lower Greensand sequence of the Baulking and Fernham areas therein (Poole & Kelk 1971; Poole et al. 1971). These investigations have demonstrated that the Lower Greensand infills valleys and depressions which possibly originated as a drainage system developed in the uplifted and eroded earlier deposits. Quartz pebbles in the lowermost Lower Greensand sands and gravels were possibly derived from exposed Triassic Pebble Beds in the English Midlands thus indicating a general uplift of all this region. Economic beds of calcium montmorillonite (fuller’s earth) occur locally at two main horizons and much of the Lower Greensand is montmorillonite bearing. This is thought to indicate contemporaneous volcanicity with deposits of trachytic ash falling on the uplifted

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