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. The main origin of the sill may be in the region of the eruptive centre that formed the Ochil Hills in a much earlier period, which geologists postulate is somewhere to the east of Stirling, possibly in the region of Kincardine. This ties in with subsidence following the first round of eruptive activity that lead to the formation of the coal basins, with a much later outbreak of intrusive activity from the same magma feeders, however the truth will remain unknown because it would take very many deep bores in hard rock to probe the depths and find the necessary evidence.