A detailed survey was carried out near Crater Lake, an unusual, circular depression 800 ft. (244 m) across, about 20 miles (~32 km) south of Yorkton, Saskatchewan. In-line and offset profiling with threefold subsurface coverage were used to outline this very small structure. The seismograms show a deep, narrow structural disturbance beneath the lake. This is interpreted as a chimney filled with breccia, which had collapsed into a solution cavern in the Prairie Evaporite salt over 3000 ft (914 m) below surface. About 125 ft (38 m) of salt has been removed from an area about 800 ft (244 m) in diameter. The chimney is about 350 ft (107 m) in mini mum diameter. Solution-collapse features are common in Saskatchewan, but previously none has been described that is so small or so well delineated on the surface. In northern Michigan, however, structures that are thought to be similar occur as columns of indurated breccia exposed by differential erosion.Small collapse structures present peculiar hazards to potash mining because they are very difficult to detect. There is also danger to roads and buildings should subsidence or sudden collapse occur in the future.

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