The BGS Lexicon of Named Rock Units — Result Details

Hwylfa Formation

Computer Code: HWY Preferred Map Code: HWY
Status Code: Pending Upgrade
Age range: Rawtheyan Stage (OW) — Hirnantian Age (OH)
Lithological Description: Grey and dark grey siltstones, muddy siltstones and subordinate parallel laminated sandy siltstones and fine- to medium-grained feldspathic sandstones. Includes locally present basal Hafod Ifan Limestone.
Definition of Lower Boundary: Base unconformable on various Caradoc formations.
Definition of Upper Boundary: top succeeded by pale grey siltstones of the Bron-Y-Graig Formation.
Thickness: 450 metres.
Geographical Limits: Not defined.
Parent Unit: Not Entered (?)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Area  Corwen, Sheet 120. 
Campbell, S D G. 1983. The geology of an area between Bala and Betws-y-Coed, North Wales. Unpublished PhD Thesis, University of Cambridge. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: