Abstract

Emar G. Poole writes: Congratulations to the authors on a most interesting paper. It was gratifying to learn that they support (p. 846) my view (Poole & Whiteman 1966, p. 17) that the mid-Cheshire Bulkeley Hill exposures demonstrate the presence of a major unconformity at the base of the Helsby Sandstone ('Keuper' Sandstone Conglomerate) which is progressively developed southwards with total erosion of the Passage Beds and the upper part of the Wilmslow (Upper Mottled) Sandstone. This unconformity is now estimated by the authors (p. 864) to have eroded some 850–900 m of strata within the basin, including the entire Wilmslow Sandstone sequence. In the marginal Stafford-Burton area, the unconformity cuts into Pebble Beds and farther south overlaps earlier Permo-Triassic deposits (Wills 1970, pp. 240, 261) eventually to rest directly upon Carboniferous in the Warwickshire-Oxfordshire Coalfield (Poole 1978, p. 13). This demonstrates a period of late Scythian compression and uplift of positive marginal areas like the London Platform and northeastern Midlands then erosion to a new base-level. In the Bicester area (western margin of the London Platform) the Helsby (Bromsgrove) Sandstone is itself overstepped by the Twyford Beds of Rhaetic age (Horton et al. 1987, p. 15, fig. 11). I support the authors' view, previously expressed by Warrington (1970, pp. 203–204) and Wills (1970, pp. 247, 261) that this unconformity correlates with the Hardegsen Unconformity of Germany. It is notable that a considerable unconformity is also developed at the base of the Blue Anchor Formation (Tea Green Mark) in the

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