The Lillooet River was dammed with breccia from a Plinian style eruption of the Mount Meager massif 2,400 years ago. The breccia damming the Lillooet River was not very strong, and the water soon eroded the breccia that was damming the river, forming Keyhole Falls. There was a massive flood when the water first broke through the breccia. The flood was big enough that small house sized blocks of breccia were carried away during the flood.