The Cassiar Mountains and Tanzilla Plateau in northern British Columbia are dominated by tuyas of the broad Tuya Volcanic Field. These flat-topped, steep-sided subglacial volcanoes were formed when magma intruded into and melted a vertical pipe in the overlying Cordilleran Ice Sheet. At least six volcanoes formed beneath glacial ice close to Tuya Lake, including Ash Mountain, South Tuya, Tuya Butte, Mathews Tuya. Ash Mountain at the head of Parallel Creek consists of pillow lava and hyaloclastite, South Tuya consists of loose volcanic debris with basaltic dikes intruding into the volcano and Tuya Butte and Mathews Tuya consist of pillow lava and hyaloclastite on its lower flanks with subaerially erupted lava flows at their flat-topped summits.