The BGS Lexicon of Named Rock Units — Result Details

Aran Boulder Bed Member

Computer Code: ARBB Preferred Map Code: notEntered
Status Code: Full
Age range: Arenig Series (OR) — Arenig Series (OR)
Lithological Description: Coarse, massive to graded volcaniclastic conglomerates and breccias comprising pebbles, cobbles and boulders predominantly of intermediate feldspar-phyric lava.
Definition of Lower Boundary: Taken at the conformable, diachronous upward passage from volcaniclastic sandstones of the Allt Lwyd Formation into conglomerate. East of the Harlech district it oversteps the lower part of the Allt Lwyd Formation to rest on strata of the Mawddach Group. The boulder bed passes distally into altered feldspathic sandstones.
Definition of Upper Boundary: Taken at the top of the Allt Lwyd Formation, where it is overlain by extrusive volcanic rocks and volcaniclastic sediments of the Benglog or Offrwm Volcanic formations. The boundary may be conformable or disconformable, represented by a condensed sequence within the Allt Lwyd Formation.
Thickness: 0 to 300m
Geographical Limits: Harlech and Bala districts.
Parent Unit: Allt Lwyd Formation (ALL)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Bryn Mawr. 
Reference(s):
Allen, P M and Jackson, A A. 1985. Geology of the country around Harlech. Memoir of the British Geological Survey, Sheet 135 with part of Sheet 149 [England and Wales]. [London: HMSO.] 112pp. 
Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovocian of Snowdonia and the Lleyn Peninsular. Research Report No.RR99/08 British Geological Survey, Keyworth, Nottingham. 40pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E135