The underlying geology of Gillies Hill is a layer of quartz-dolerite approximately 100 meters thick of the Late Carboniferous tholeiitic Midland Valley Sill-complex (Stirling Sill). The hill is one of a series of crag and tail formations that cross the Carse of Stirling separated by faults into distinct hills such as Abbey Craig, Kings Park, Sauchieburn and Stirling Castle rock. These crag and tail formations were created by glaciation that terminated about 10,000 years ago. The quartz-dolerite of the hill, highly resistance to erosion, is a ready source of whin-stone for road beds and sea walls; concrete aggregate and kerbstones. Several whinstone mines have operated on Gillies Hill over the years.