The BGS Lexicon of Named Rock Units — Result Details

Thornylee Formation

Computer Code: TLEE Preferred Map Code: THL
Status Code: Full
Age range: Llandovery Epoch (SL) — Llandovery Epoch (SL)
Lithological Description: Interbedded turbidite (wacke) sandstone and mudstone/siltstone passing locally to massive sandstone. Sandstone typically fine- to medium-grained but ranging to coarse-grained especially in massive units; commonly medium- to very thick bedded (up to 2m) with sparse units up to a few metres thick that are very-thin to thin-bedded, usually with thin siltstone or silty mudstone interbeds; massive sandstone units tens of metres thick and massive siltstone units a few metres thick occur locally. Sandstone and siltstone are typically grey to greenish grey when fresh, weathering olive green to pale brown. The sedimentological character is transitional from the Gala Group to the north, but the Thornylee Formation is distinguished by its younger age range, abundant garnet in coarse-grained sandstone and relatively low Sr and Ni content in whole rock sandstone composition. To the south, the adjoining Selcoth Formation (Ettrick Group) lacks the very thick massive units of coarse-grained sandstone and siltstone and generally has a higher proportion of interbedded mudstone; garnet is rare in the Selcoth Formation. However, the Thornylee and Selcoth formations are the same age and have similar whole rock sandstone composition; they are therefore closely related if not facies variants.
Definition of Lower Boundary: Conformable sedimentary contact with rocks of the Moffat Shale Group.
Definition of Upper Boundary: Faulted, none preserved.
Thickness: Uncertain, likely to be at least several hundred metres.
Geographical Limits: Southern Uplands, Scotland.
Parent Unit: Ettrick Group (ETRK)
Previous Name(s): Garnetiferous Group (-1534)
Gala 7 (-2487)
Queensberry Grit (-2488)
Buckholme Formation (-1866)
Queensberry Formation (QBY)
Alternative Name(s): none recorded or not applicable
Type Area  Craggy upland exposure and exposure in forest tracks and small quarries, Thornyleee Crags, railway cutting (disused) east of Thornylee Farm adjacent to A707, 4 to 5km east of Walkerburn. Walton, 1955: Kassi and Weir, 1993 
Reference Section  River section in Dryhope Burn, extending 2.5km northnorthwest from Dryhope at the eastern end of St Mary's Loch. 
Kassi, A M and Weir, J A. 1993. Depositional and geotectonic history of the Gala area, eastern Southern Uplands, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol.84, 161-173. 
Walton, E K. 1955. Silurian greywackes in Peeblesshire. Proceedings of the Royal Society of Edinburgh, B65, 327-357. 
British Geological Survey. 1996. Peebles. Scotland Sheet 24E. Solid geology. 1:50 000 (Keyworth, Nottingham: British Geological Survey.) 
Peach, B N and Horne, J. 1899. The Silurian Rocks of Britain, Volume 1. Scotland. Memoir of the Geological Survey of the United Kingdom. 
Webb, B C, Rushton, A W A and White, D E. 1993. Moffatdale and the upper Ettrick valley. Description of the solid geology of parts of 1:25000 sheets NT10, 11, 20 and 21. Classical Areas of British Geology. (London: British Geological Survey, HMSO.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable