The Bridge River Vent is a relatively young volcanic crater that formed during an eruption about 2,400 years ago. This eruption ranged in character from explosive to effusive and involved lava dome extrusion, pyroclastic flows, lahars and lava flows. Eastward migration of the eruption column spread material across Western Canada to deposit the Bridge River Ash. In the Bridge River and Lillooet River area the ash occurs as a coarse-textured deposit with blocks of pumice up to in diameter. The texture rapidly becomes finer eastward from the Bridge River. At Big Bar on the Fraser River pellets are up to in diameter while pellets in the Messiter area have a maximum diameter of .