The BGS Lexicon of Named Rock Units — Result Details

Rodway Siltstone Formation

Computer Code: RSI Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Marsdenian Substage (CZ) — Yeadonian Substage (CY)
Lithological Description: Dark grey, locally red, purple and green stained, micaceous siltstone with subordinate thin beds of pale grey or purple-stained, fine- to medium-grained, locally bioturbated sandstone. Near the base there are small-scale rhythmic cycles of sandstone, siltstone and black shale.
Definition of Lower Boundary: At the type locality the base of the formation is faulted against mid to pale grey oolitic limestone of the Pembroke Limestone Group. A non-tectonized base of the Formation has not been recorded at crop or in boreholes, suggesting that the formation was displaced to its current location above a low-angle lag fault, (Whittaker, 1975).
Definition of Upper Boundary: The grey siltstones of the Rodway Silstone Formation are overlain unconformably by red, yellow and buff mottled, fine- to medium-grained sandstone of the Otter Sandstone Formation (Sherwood Sandstone Group).
Thickness: More than 138m, with 138m exposed (top not seen) at the type locality.
Geographical Limits: The outcrop is restricted to the Rodway Inlier [ST 244 398] and Swang Farm Inlier [ST 231 392], located northwest of Cannington, Somerset. The Formation is also proved in boreholes to occur beneath the Sherwood Sandstone and Mercia Mudstone groups within the vicinity of the inliers.
Parent Unit: Marros Group (MARR)
Previous Name(s): Rodway Siltstones (-3650)
Rodway Beds (ROS)
Alternative Name(s): none recorded or not applicable
Partial Type Section  The Withiel Farm No.1 Borehole (ST23NW/6) [ST 2435 3981], near Cannington, Bridgwater, proved the thickest development of the formation from ground surface to 138.2m depth and records the base. Edwmonds and Williams, 1985. 
Whittaker, A. 1975. Namurian strata near Cannington Park, Somerset. Geological Magazine, Vol.112, 325-326. 
Edmonds, E A and Williams, B J, 1985. Geology of the country around Taunton and the Quantock Hills. Memoir of the British Geological Survey, Sheet 295. (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used: