The pilot was mustering over sandstone which was laced with shallow gorges and gullies. After sensing a shudder and engine miss, the pilot elected to carry out an immediate precautionary landing and landed on an outcrop of rock. While still at high RPM, the pilot moved the pedals and cyclic control to ensure that the skids were on a firm footing and tightened the collective friction. However, shortly after he exited the aircraft to inspect the idling engine, the helicopter pitched rearwards and slid off the rock into the gorge.