Gargunnock Hills Lava Member
Gargunnock Hills Lava Member (GHLA), Fintry-Touch Block, Carboniferous, Midland Valley of Scotland
Gargunnock Hills Lava Member is part of the Clyde Plateau Volcanic Formation.
Previously named the Gargunnock Hills Group (Francis et al., 1970).
The Gargunnock Hills Lava Member consists predominantly of plagioclase-macrophyric basalt (‘Markle’ type) and composite lavas, with subordinate plagioclase-microphyric basalt (‘Jedburgh’ type) and rare basaltic-trachyandesite (mugearite) lavas. The topmost part of the member is well exposed in the gorge of the Touch Burn, downstream from the waterfall of Gilmour’s Linn (NS 7395 9252) where there are three massive to variably ‘slaggy’, plagioclase-macrophyric basalt lavas, with weathered tops (dark red boles with nodules of decomposed, ‘slaggy’ basalt). The remainder of the member is reasonably well exposed on the northern and eastern slopes of Scout Head (NS 7354 9337 to NS 7350 9313) where plagioclase-microphyric basalt lava flows (several of which are transitional to plagioclase-macrophyric basalt), and plagioclase-macrophyric basalt basalt lava flows are present.
The type area is in the Gargunnock and Touch hills, and the northern Kilsyth Hills, north-east of Glasgow (NS 6970 8456 to NS 7515 9360) (Francis et al., 1970). Partial type sections occur in the Touch Burn (NS 7395 9252) and on the northern and eastern slopes of Scout Head, northern Touch Hills, north-east of Glasgow (NS 7354 9337 to NS 7386 9301) (Francis et al., 1970). It is 75 m thick in all but the topmost flows of the member.
Lower and upper boundaries
In the northern Gargunnock Hills, the basal plagioclase-macrophyric basalt flow of the member lies, apparently disconformably, on trachybasalts of the Lees Hill Lava Member. In the northern Touch Hills, the Gargunnock Hills Lava Member interdigitates with the Lees Hill Lava Member and lies, probably disconformably, on microporphyritic basalts (‘Jedburgh’ type) of the Spout of Ballochleam Lava Member. In the eastern part of the Kilsyth Hills, Campsie Block, the Gargunnock Hills Lava Member overlies the Langhill Lava Member, but the nature of the contact is unknown. In the north-eastern Kilsyth Hills the member overlies, probably unconformably, the Campsie Lava Member.
In the Gargunnock Hills and north-western Touch Hills, the member occupies the highest ground and its top is eroded. However, in the north-eastern and eastern Touch Hills, the member is overlain by the Black Mount Lava Member. The change in lithology is from plagioclase-macrophyric basalt to plagioclase-microphyric basalts and subordinate basaltic trachyandesite (mugearite).
From 91 m in the east, to more than 150 m in the west, where the top of the member is eroded.
Distribution and regional correlation
The member is restricted to the Fintry–Touch Block (Francis et al., 1970) and the north-eastern part of the Campsie Block (Forsyth et al., 1996) and specifically to the north-eastern Kilsyth Hills, and the Gargunnock and Touch hills, north-east of Glasgow. These rocks crop out in the north-eastern Kilsyth Hills on the upper parts of Cairnoch Hill (NS 697 857), Craigannet Hill (NS 712 848) and Dundaff Hill (NS 735 844) and over much of the area to the north of Shielwalls Fault (Craig, 1980), including Craigengelt Hill (NS 7240 6852), Earl’s Hill (NS 7175 8842) and Cringate Law (NS 6834 8830), and the area to the north, south and west of Loch Coulter (NS 764 860). The rocks also crop out extensively across the upper parts of the Gargunnock Hills, from Burnfoot (NS 675 883) in the south-east, north-north-east to Carleatheran (NS 6877 9186) and east, across the Touch Hills to Craigniven (NS 7515 9360).
Mid Visean (Arundian to Asbian).
- Francis, E H, Forsyth, I H, Read, W A, and Armstrong, M. 1970. The geology of the Stirling district. Memoir of the Geological Survey of Great Britain, Sheet 39 (Scotland)
- 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