The BGS Lexicon of Named Rock Units — Result Details

Tollhouse Mudstone Bed

Computer Code: THM Preferred Map Code: notEntered
Status Code: Full
Age range: Asbian Substage (CR) — Asbian Substage (CR)
Lithological Description: Fossiliferous, dark grey, calcareous mudstone and highly argillaceous limestone. The Tollhouse Mudstone is a widely recognised bed of marine mudstone occurring within the late Asbian platform carbonate succession of North Wales. Recognised on Anglesey and at Llandudno, it appears to record a temporary cessation of limestone deposition and (or caused by) an increase in the volume of terrigenous mud supplied to this northwestern portion of the platform.
Definition of Lower Boundary: In its type section, the base is taken at the contact between the mudstone and underlying dolomitised limestones (formerly part of the Llandudno Pier Dolomite, but now included in the Loggerheads Limestone Formation).
Definition of Upper Boundary: In its type section, the top is taken at the entry of overlying, pale, thick-bedded limestones.
Thickness: To 2m
Geographical Limits: Anglesey (SJ 50 87) and the Great Orme (SJ 77 83), northwest Wales.
Parent Unit: Loggerheads Limestone Formation (LGHL)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  West side of Marine Drive, 100m NNW of Tollhouse, Llandudno; and on the foreshore below, where the bed is exposed in its entirety. Warren et al., 1984 
Warren, P T, Price, D, Nutt, M J C and Smith, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: