Gloucestershire Geology Trust


Sidmouth Mudstone

Mid-Late Triassic (Anisian-Carnian)

Mercia Mudstone
Sidmouth Mudstone

The formation consists of dominantly mudstone and siltstone, red-brown with common grey-green reduction patches and spots. The mudstones are mostly structureless, with a blocky weathering habit, but intervals up to 15m thick of interlaminated mudstone and siltstone occur within parts of the formation. Heterolithic units consisting of several thin beds of grey-green dolomitic siltstone and very fine-grained sandstone, interbedded with mudstone, occur at intervals throughout the formation. These units range from a few centimetres up to 4m thick. Breccias produced by contemporaneous dissolution of salt are common throughout the formation. Gypsum/anhydrite also occurs throughout the formation as nodules and veins.

Upper Boundary
Placed at a rapid upward transition from red mudstones or siltstones to the dominantly grey or green, partly or largely arenaceous beds of the overlying Arden Sandstone Formation. At outcrop, the boundary is typically marked by a change from the reddish-brown clayey soils of the Sidmouth Mudstone Formation to the grey, slightly sandy clay soils of the overlying Arden Sandstone Formation. Where the latter forms a marked cuesta feature, the boundary typically lies at a slight concave break in the scarp slope.

Lower Boundary
The boundary is both gradational and conformable, and is marked by a rapid upward transition from sandstone- to mudstone-dominated lithologies immediately above the Sherwood Sandstone Group. At outcrop, the boundary is associated with a change from the brown sandy soils of underlying formations to the clayey, distinctively reddish brown soils of the Sidmouth Mudstone Formation.

Representative Sites
Mugglewort Cutting

Information on lithology and boundaries taken from the British Geological Survey Lexicon of Named Rock Units

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