The BGS Lexicon of Named Rock Units — Result Details

Scrach Formation

Computer Code: SRA Preferred Map Code: Sra
Status Code: Pending Upgrade
Age range: Hirnantian Age (OH) — Hirnantian Age (OH)
Lithological Description: Mudstones with numerous thin sandstones; packets of thick-bedded sandstone and conglomerate locally present. Flaser and lenticular bedding common in thin sandstones. NOTE: In type area overlain by Bronydd, Coldbrook or Goleugoed Formations and underlain by Tridwr Formation. In the Rhayader district overlies the Yr Allt Formation. Top not seen.
Definition of Lower Boundary: Gradational with underlying Tridwr or Yr Allt Formations. Taken at entrance of abundant thin sandstones.
Definition of Upper Boundary: Sharp entrance of mudstone dominated sequences of the Bronydd, Coldbrook or Golengoed Formations in the type area. Top not seen at Rhayader due to local disconformity at base of Telychian Formations.
Thickness: Up to 150 metres.
Geographical Limits: Central Wales.
Parent Unit: Not Entered (?)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  Forestry cutting in Crychan Forest, north east of Llandovery. See fig.3 of Cocks, and others, 1984. 
Woodcock, N H, and Smallwood, S D. 1987. Late Ordovician Shallow Marine Environments due to Glacio-Eustatic Regression, Scrach Formation, Mid-Wales. Quarterly Journal of the Geological Survey, Vol.144 (3), p.393-400. 
Cocks, L R M, Woodcock, N H, Rickards, R B, Temple, J T, and Lane, P D. 1984. The Llandovery Series of the type area. Bulletin of the British Museum (N.H.), Geology, Vol.38, p.131-182. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: