[1] The sill is of very late Carboniferous age or more probably Permian, as it penetrates the coal measures, often in bedding planes between the various strata.
At its outcrop on the west, the sill generally lies towards the base of the Carboniferous sequence, and just above the calciferous sandstone deposits and lavas, as can be seen in the upper part of the valley of the Bannock Burn.
The slope of the Abbey Craig, or the Stirling Castle rock, gives a general idea of the angle of dip of the coal measures at the extremity or the coalfield, and the thickness of the sill can also be seen, which is approximately 100 metres.
[citation needed] The source of the sill, where the molten rock rose from below, is not known; however it is known that there are risers in the Ochil Fault, although these are inconsistent with the overall structure.
[citation needed] The western extremities of the sill can be seen at the Abbey Craig, Stirling Castle, Kings Park, Gillies Hill, Sauchieburn and various points to the south.