Drumwhirn Member is part of the Ballagan Formation.
The member was previously referred to by Eyles et al. (1949) as the ‘Basal Sandstone’ of the Cementstone Group of the Calciferous Sandstone Measures. The present unit was named after Drumwhirn Farm and defined by Monaghan (2004).
White-buff and yellow, medium- to fine-grained, thinly planar-bedded sandstone intercalated with finely laminated grey calcareous siltstone, calcareous mudstone, grey silty mudstone and rare, thin, nonmarine, muddy ferroan dolostone (‘cementstone’).
The type area is in the hills east and south of Lindsayston Burn south-east of Dailly between Craig and Drumwhirn farms and east of Whitehill Farm (NS 275 005 to 300 016) (see Eyles et al., 1949; Floyd, 1999; Monaghan, 2004).
Lower and upper boundaries
The basal contact is faulted in the type area (see above). The presence of grey silty mudstone and dolomitic limestone (‘cementstone’) interbedded with sandstone distinguishes this basal member of the Ballagan Formation from the underlying sandstone-dominated Kinnesswood Formation (Figure 6, Column 4B).
The upper boundary is mapped as a conformable contact on 1:10.000 scale Geological Sheet NS20SE in the Lindsayston Burn section at (NS 2824 0080) but is not actually exposed. The boundary is defined as the lithological change from yellow sandstone with grey silty mudstone to red-purple sandstone with conglomerate and pedogenic carbonate typical of the Lindsayston Burn Member.
The member is at least 275.m thick in the type area (see above), but the base is faulted. The unit is interpreted to be of laterally variable thickness.
Distribution and regional correlation
The member occurs on 1:50.000 scale Geological sheets Scotland 14W (Ayr) and Scotland 8W (Carrick), and 1:10.000 scale Geological sheets NS20SE and NS30SW (part of) near Dailly, South Ayrshire. The lateral extent of the unit is not yet finalised.