Sinkholes are very common in the bench lands east of Kamloops, and can be fascinating surprises for the hikers who come upon them. This is the story of how they form.
Near the end of the era of massive glaciers the Thompson River valley was submerged under a large glacial lake, and the bottom of this lake, from Kamloops to Pritchard, was composed of a layer of silt up to 100 meters (330 feet) thick. Today this silt forms well-defined benches on either side of the river which are cut by an extensive system of gullies.
Silt is a stable material when dry, but is weak and easily eroded when saturated with water, and because of this is very susceptible to the formation of sinkholes. Sinkholes are formed by a type of subsurface erosion called piping: water percolates into the silt during infrequent storms or spring run-off until it reaches a temporary water table, then moves horizontally until reaching a gully wall.
Here the wet silt collapses, or caves, but the dry silt above remains stable and forms an arch over the caved area, creating an opening in the face of the wall. Continued seepage from the area where the caving began extends the opening back into the wall, forming a tunnel. When caving sufficiently weakens the silt arch at some point in the tunnel, it collapses and a depression, a sink, appears on the surface above. The process continues following this collapse, because the sink now acts as a collecting basin for surface water and the disturbed silt below it provides an easy path down to the pipe.
Collapse from caving can happen at any point in a pipe, and it often takes place simultaneously at several different points. When this occurs a chain of sinks will appear. Over time these sinks enlarge as the erosion continues, and eventually so much silt is removed that they connect and a new gully forms.
Sink holes can occur as cave-like openings in the walls of gullies, vertical shafts at the bottoms of cone-shaped depressions both on the surface of the benches or in gully bottoms, or downward-angling holes that connect one sink in a chain to the next.
For hikers trying to find sink holes in gully bottoms, look for patches of green shrubbery or tall grass that stand out from their surroundings; at the heads of gullies look for isolated trees in otherwise open terrain. Trees frequently grow in or immediately beside sink holes.