Cleal, C.J. & Thomas, B.A. 1996 British Upper Carboniferous Stratigraphy. Geological Conservation Review Series No. 11, JNCC, Peterborough, ISBN 0 412 72780 3.
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The cliff on the River Tonge at Mill Hill is the best available exposure of the Peel Hall Rock
This cliff on west side of River Tonge
The cliff here exposes some 12 m of sandstone from the lower part of the Peel Hall Rock Formation, overlying grey mudstones. The sandstone, which is pale grey in colour and fine-grained, shows some cross-bedding. However, the two-dimensional nature of the face makes it impossible to determine a meaningful palaeocurrent direction.
The sandstones here contain no fossils, other than indeterminable plant fragments. At the type locality for the formation (Peel Hall Quarry, Little Hutton, Greater Manchester) there have been records of numerous casts of Trigonocarpus seeds (Tonks et al., 1931, pl. 2), but these are of no biostratigraphical value.
The mudstones underlying the sandstone here were reported by Tonks et al. (1931) to contain bivalves, including Anthracosia cf. phryglana (Wright). This suggests the subzone of the same name, indicating the lower Duckmantian.
This is the best exposure of the Peel Hall Rock, a fluvio-deltaic, arenaceous formation in the lower Duckmantian of the Lancashire Coalfield. It reaches a maximum of 45 m thick in this part of the coalfield, but thins rapidly to the south and eventually disappears at about Tyldesley. As well being an important fluvio-deltaic complex, the formation is of interest for providing of the distinctive Trigonocarpus 'nuts' found in many museum collections.
The River Tonge at Mill Hill is the best available exposure of a sandstone known as the Peel Hall Rock, about 305 million years old.