The area is underlain by basalt flows, agglomerates, and tuffs of the Tertiary Masset Formation. The general strike of the flows is 170 degrees with moderate southwest dips.

Bitumen (tar) occurs in veins and amygdules within the basalts and agglomerates. The cavities commonly contain quartz, chalcedony, calcite and the bitumen. The veins are up to a metre in width and occasionally a few metres long.

J.D. Mackenzie (Geological Survey of Canada Memoir 88) believes that the bitumen has an organic origin being absorbed from underlying sediments such as the bituminous argillites of the Jurassic Maude Group. Athol Sutherland Brown (Bulletin 54) agrees, stating that the Masset flows at Tian Point overlie sandstone or shales that contain much woody matter and therefore the tar has its origin as a wood distillate.