Crescent-shaped glacial erosional mark carved down-current of an obstacle of greater mechanical resistance than the host rock it is encassed in. Thought to be generated by the erosive action of turbulent sediment-ladden meltwater flow. The structure is named after the elongated positive relief in the center of the eroded crescent which is reminescent of a rat tail due to its elongated and tapered shape. Spectacular examples of these structures can be observed locally at a quarry in Cantley (Qc).