Mudrocks occur as thin graptolitic shales, as shelf deposits, and within thick turbidite sequences. It is the thick turbidites sequences that provide the most extensive areas of outcrop, especially in the Grampian Highlands, the Lake District and the Welsh Basin. Thin shale bands in the Southern Uplands can provide engineering hazards. Shelf muds are common in the Tremadoc and Silurian shelf sequences of England. Reconstructions of the contemporary palaeogeography suggest possible source areas in Scotland, the Irish Sea landmass and the East Midlands of England for some of the Lower Palaeozoic sedimentary sequences, but others are of unknown provenance.

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