The Thumb is a mountain located 7 km (4 mi) south of Sitchiada Mountain on the east side of Bear Lake, on the divide between the upper Omineca River and the basin of the Bear River in the Omineca Country of the Central-North Interior of British Columbia, Canada.

They are surrounded by the remains of eroded cinder cones, lava flows and dikes.

Even though the plugs have not been dated, the existence of loose scoria and related intravalley lava flows to the current topography indicates they formed in the last 2.5 million years of the Quaternary period.

[1] The Thumb is largely made of columnar basalt bounded by pockets of breccia comprising clasts of the basal sandstone that formed during the Paleocene period.

The Thumb consists of alkali olivine basalt along with other Quaternary volcanic plugs in the Omineca Mountains.