The BGS Lexicon of Named Rock Units — Result Details

Yr Allt Formation

Computer Code: YA Preferred Map Code: YA
Status Code: Pending Upgrade
Age range: Ashgill Series (OA) — Ashgill Series (OA)
Lithological Description: Very silty mudstones with abundant siltstone laminae and scattered thin beds of sandstone. Units of thick-bedded sandstone locally present. Slump folding and destratification commonly present.
Definition of Lower Boundary: Sharp incoming of mudstones with siltstone laminae and no burrows, above burrow mottled mudstones of the Nantmel Formation. Locally rests sharply on top of Doldowlod Conglomerate Member of Nantmel Formation.
Definition of Upper Boundary: Either gradational and taken at incoming of sandstone dominant sequence of Scrach Formation, or sharp, where overlain by mudstones lacking silt laminae of the Cwmere Formation or Tycwtta Mudstones.
Thickness: Up to 1200 metres.
Geographical Limits: Central Wales.
Parent Unit: Not Applicable (-)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  Road section on A470 at base of hill, known as Yr Allt, north of Newbridge on Wye. 
none recorded or not applicable
1:50K maps on which the lithostratigraphical unit is found, and map code used: