A kolk (colc) is an underwater vortex created when rapidly rushing water passes an underwater obstacle in boundary areas of high shear. High velocity gradients produce a violently rotating column of water, similar to a tornado. Kolks can pluck multi-ton blocks of rock and transport them in suspension for thousands of metres.
Kolks leave clear evidence in the form of plucked-bedrock pits, called rock-cut basins or kolk lakes and downstream deposits of gravel-supported blocks that show percussion but no rounding.(Wikipedia)