The BGS Lexicon of Named Rock Units — Result Details

Thorganby Clay Member

Computer Code: THOR Preferred Map Code: notEntered
Status Code: Full
Age range: Devensian Stage (QD) — Devensian Stage (QD)
Lithological Description: Greyish brown soft, locally fissured, laminated silt and clay. The top metre of this member commonly contains a higher percentage of silt and sand, is reddish yellow to grey colour-mottled and the laminated structure has been destroyed by periglacial and soil processes. This unit can contain gravel dropstones and glaciotectonic structures, particularly near the Escrick Moraine.
Definition of Lower Boundary: The Thorganby Clay Member is shown in boreholes and sections to rest directly on the Lawns House Farm Member. The contact can be sharp or gradational with interbedded sand, silt and clay layers. The Thorganby Clay Member only occurs where the Lawns House Farm Sand Member is present. Outside of this area, the equivalent clay is part of the undivided Hemingbrough Formation.
Definition of Upper Boundary: The Thorganby Clay Member is typically exposed at surface, or concealed beneath the Breighton Sand Formation. The upper boundary may not be preserved where Skipwith Peat Bed has formed directly on silted up or dried up lake bed. Where the base of the overlying Breighton Sand Formation is represented by sandy facies, the upper boundary is typically sharp and erosive. In most places the upper boundary is disturbed by periglacial and soil processes.
Thickness: 1 to 3.5m. (Proved in auger holes).
Geographical Limits: The Thorganby Clay Member occurs south of the Escrick Moraine [SE703 450] between the rivers Ouse and Foulness. In south of district deposit is likely to merge with other members of of the Hemingbrough Glaciolacustrine Formation.
Parent Unit: Hemingbrough Glaciolacustrine Formation (HEM)
Previous Name(s): Silt And Clay Of 25 Foot Drift (-567)
Sand Of 25 Foot Drift (-1229)
Alternative Name(s): Warp
Stratotypes:
Partial Type Section  Ditch section 250m north of Gale Farm, about 1200m westsouthwest of Thorganby. Shows 0.5m of stiff greyish brown laminated clay and silt of the Thorganby Clay Member. This is covered by 1.0m of mottled clayey silty sand, which is believed to resemble the active layer, that has admixed coarser material. 
Reference(s):
British Geological Survey, 1973. Sheet 71 Selby solid and drift/compiled and draw in Leeds Office of the Institute of Geological Sciences - Solid and drft ed. Provisional ed. 
1:50k geological map of Selby (Sheet 71) (in prep): Selby Sheet Explantion (in prep). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable