British Columbia
Southwestern British Columbia

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Keyhole Falls, Mount Meager Volcanic Complex, southwestern British Columbia.

Keyhole Falls is located at the head of Lillooet Canyon, which formed within the volcanic rocks occurring in Lillooet Valley, adjacent to the Mount Meager Volcanic Complex (a cluster of closely-spaced volcanic centres). The upstream (or headward) retreat of the waterfalls has formed Lillooet Canyon. The presence of the falls relates to the presence of resistant, welded pyroclastic rocks, overlying weaker, poorly welded pyroclastic rocks. Erosion of the softer rock at the base of the falls undermines the more resistant rock, allowing the falls to cut the canyon. The narrow slit above the waterfalls has formed through erosion into the resistant volcanic rock along the riverbed. This process is much less efficient than undermining; thus, over the same time period, the waterfall has retreated several hundreds of metres and the river has vertically eroded the channel by a few tens of metres. Keyhole Falls and the volcanic rocks forming the canyon are geologically very young. The rock was formed by a major volcanic eruption at the Mount Meager Volcanic Complex in 411 BC.

Latitude: 50.675
Longitude: -123.454
Physiographic Region: Information not available
Photographer: Greg Brooks
GSC Photo Number: 2002-693
High Resolution Image: Available on request

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