The BGS Lexicon of Named Rock Units — Result Details

Henllan Ash Member

Computer Code: HNA Preferred Map Code: HNA
Status Code: Pending Upgrade
Age range: Arenig Series (OR) — Arenig Series (OR)
Lithological Description: Light grey feldspathic sandstones. Rare large-scale cross-bedding. Locally muddy and strongly bioturbated. Rich shelly fauna.
Definition of Lower Boundary: Base contrasts with flaggy laminated siltstones and sandstones below.
Definition of Upper Boundary: Top marked by incoming of mudstone.
Thickness: 190 metres. Lateral variations 20 - 190 metres.
Geographical Limits: Not defined.
Parent Unit: Allt Lwyd Formation (ALL)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Area  Bala, Sheet 136. 
Fearnsides, W G. 1905. On the geology of Arenig and Moel Llyfnant. Quarterly Journal of the Geological Society, Vol.61, 608-640. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: