Overton Lava Member
Overton Lava Member is part of the Clyde Plateau Volcanic Formation.
Previously named the Overton Lavas (Forsyth et al., 1996).
The Overton Lava Member consists of at least one plagioclase-macrophyric basalt lava (‘Markle’ type) overlying a basaltic-trachyandesite lava (mugearite). Several lava flows, including trachybasalt, may be present. No source for the lavas has been identified.
The type area is the eastern Kilsyth Hills, north-east of Glasgow (NS 7532 8120 to NS 7706 8434) (Forsyth et al., 1996). Best exposures are in the faulted River Carron section, to the north-west of Nicholswalls (NS 764 843) and in the reference section in the Overton Burn (NS 7745 8353 to NS 7673 8344), east of Overton, eastern Kilsyth Hills (Craig, 1980).
Lower and upper boundaries
The basal basaltic-trachyandesite (mugearite) lava of the member is conformable or disconformable in the north on the trachybasalt lavas of the undivided lower north Campsie and lower south Campsie lavas (Campsie Lava Member). Elsewhere the base is not seen.
The Overton Lava Member in the east is overlain conformably or disconformably by the undivided upper north Campsie and upper south Campsie lavas (Campsie Lava Member), and in the north the member is overlain specifically by the upper north Campsie lavas (Campsie Lava Member). The transition is from plagioclase-macrophyric basalt lava to plagioclase-microphyric trachybasalt and some basalt lava (basalt to hawaiite of ‘Jedburgh’ type) with scoriaceous agglomerate and tuff.
Some 35 m.
Distribution and regional correlation
The member is restricted to the Campsie Block (Forsyth et al., 1996) and specifically to the eastern part of the Kilsyth Hills, north-east of Glasgow. These rocks crop out in two main areas. The first and principal area is in faulted ground to the south, east and north of Darrach Hill (NS 7537 8276), from the east of Doups (NS 7533 8122), east to near Meadowgreens (NS 7765 8297), north to Langhill (NS 7706 8432) and a stream east of Glenhead (NS 759 852), with a small outlier west of the River Carron (NS 7529 8460). The second area is a small outlier in the Earl’s Burn (NS 7175 8597 to NS 7129 8650).
Mid Visean (Arundian to Asbian).
- Forsyth, I H, Hall, I H S, and McMillan, A A. 1996. Geology of the Airdrie district. Memoir of the British Geological Survey, Sheet 31W (Scotland)
- Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster