The BGS Lexicon of Named Rock Units — Result Details

Gregynog Mudstone Member

Computer Code: GYM Preferred Map Code: GYM
Status Code: Full
Age range: Sheinwoodian Age (SS) — Homerian Age (SH)
Lithological Description: Mainly mudstone, silty medium-dark grey homogeneous, slumped and in part thinly interbedded with thin fine-grained sandstones.
Definition of Lower Boundary: Diachronous, upon the greywacke sandstones of the Denbigh Grits/Penstrowed Grits.
Definition of Upper Boundary: Where the bedding becomes prominent in rhythmic ribbon banded mudstones and siltstones of the Nantglyn Flags type. In practice, at the base of the Mottled Mudstone Member.
Thickness: At least 130m.
Geographical Limits: So far 1:50 000 map, Sheet 165 (Montgomery), western edge.
Parent Unit: Nantglyn Flags Formation (NGF)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Reference Section  Henglog Dingle. 
Type Section  To be designated - a better section probably exists on adjacent, but unsurveyed ground. 
none recorded or not applicable
1:50K maps on which the lithostratigraphical unit is found, and map code used: