Mount Brew is a volcanic feature in the Mount Cayley volcanic field of the central Garibaldi Volcanic Belt of the Canadian Cascade Arc. It formed during the Pleistocene period when volcanic activity in this area occurred under glacial ice during the last glacial period. Although it had a vigorous start, the eruption that formed Mount Brew was not sufficiently sustained to form a larger edifice that could break through the surrounding ice and water to form a tuya. Instead, the eruption produced a subglacial mound.