ABSTRACTA large talus-foot debris accumulation on Muckish Mountain is interpreted as a fossil rock glacier rather than a glacial moraine as previously suggested. Sedimentological characteristics indicate that the debris accumulated initially as talus. Rock glacier flow probably occurred due to the deformation of interstitial ice and ice lenses. A bedrock ridge at the talus-foot was almost totally buried by rock glacier flow and now exaggerates the height and thickness of the fossil rock glacier. The major stages in the development of the fossil rock glacier are outlined in a simple model. Debris accumulation is believed to have occurred during the Lateglacial period with rock glacier movement taking place in the Nahanagan Stadial.