Keyhole Falls was formed when the Lillooet River was dammed with breccia from a Plinian eruption at the Mount Meager massif about 2350 years ago. The thick breccia soon eroded from water activity, 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.