Overtoun Sandstone Member
Overtoun Sandstone Member (OVS), North of the River Clyde, Carboniferous, Midland Valley of Scotland
Overtoun Sandstone Member is part of the Clyde Sandstone Formation.
The lower part of the Overtoun Sandstone Member, approximately 36 m thick, is dominated by grey, fine-grained, thin-bedded or partly concretionary sandstone. There are also intervals of mudstone, up to 3.3.m thick, commonly with calcareous concretions and uncommon beds of concretionary limestone or dolomitic limestone (‘cementstone’). The upper part, approximately 20.m thick, largely comprises pale grey, mostly fine-grained sandstone with clasts of carbonate. Unlike the lower part of the member, sandstones with concretions and mudstones are uncommon.
The type section is the BGS Barnhill Borehole, near Overtoun (BGS Registration Number NS47NW/2) (NS 4269 7571) from 104.89 to 161.48 m depth.
Lower and upper boundaries
The lower boundary of the member is conformable and overlies strata of the Ballagan Formation that are characterised by grey mudstone and dolomitic limestone (‘cementstone’). In the Barnhill Borehole (see above) the base is picked, at 161.48 m depth, at the top of a unit dominated by grey mudstone with gypsum veins, overlain by a sequence dominated by thick packages of sandstone.
In the same borehole at 104.89 m depth, the upper boundary of the Overtoun Sandstone Member is sharp and picked at a marked change from grey sandstone and mudstone to grey tuff, with volcanic lapillae, of the overlying Clyde Plateau Volcanic Formation. Correlation within the Greenock district demonstrates that the base of the Clyde Plateau Volcanic Formation is unconformable.
Some 56.6 m in the type section (see above).
Distribution and regional correlation
Area around Overtoun House, north of the River Clyde near Dumbarton, Strathclyde.
- Institute of Geological Sciences. 1978. Institute of Geological Sciences boreholes 1977. Report of the Institute of Geological Sciences, No. 78/21
- Paterson, I B, Hall, I H S, and Stephenson, D. 1990. Geology of the Greenock district. Memoir of the British Geological Survey, Sheet 30W part 29E (Scotland)