At Avalanche Lake, located in the Backbone Ranges of the Mackenzie Mountains, about 200 x 10(6) m(3) of massive Devonian carbonate rock slid down remarkably planar bedding surfaces dipping at 30 degrees and created a spectacular runup on the opposite valley side onto a topographic feature called the Shelf. The interpretation of events at Avalanche Lake has recently been subject to controversy. It has been argued by other workers that the rock avalanche could not have run onto the Shelf without glacier ice partially filling the valley, thus reducing the magnitude of the actual runup, and implying that the rock avalanche took place at the end of the Pleistocene. Evidence is presented indicating that the rock avalanche occurred in an ice-free environment. It consists of the nature of the detachment surface, the morphology and location of the rock avalanche debris, the presence of levees in the debris and isolated patches of debris on valley-side slopes, and the entrainment of alluvial deposits and conifer fragments from the valley floor in the Shelf Lobe debris. In addition, radiocarbon ages obtained from entrained wood in the debris, converted to calendric years, indicate that the landslide took place in this millennium, with a 95% probability of it having occurred no earlier than 1440 A.D. No glacier ice then existed in the valley. Based on this evidence the behaviour of the rock avalanche is reconstructed. It is characterized by dramatic mobility in which the rock avalanche split into two parts. The west part smashed into the opposite valley side and about 5 X 10(6) m(3) rode up onto the Shelf. The remainder (155 x 10(6) m(3)) fell back into the valley, partially running back up the detachment surface to an elevation 360 m above the valley, and then, reversing direction again, ran back into the valley bottom where it was deposited. The east part, the South Lobe (40 x 10(6) m(3)), ran down a valley reentrant opposite the detachment surface. The maximum vertical drop in the path is 1220 m, and the maximum runup is 640 m. The fahrboschung is 8 degrees for the Shelf Lobe and 10 degrees for the South Lobe. An analysis of the movement of the centre of gravity using a version of Koerner's dynamic model simulates the runup onto the Shelf, indicating that the presence of glacier ice is not necessary to account for the runup magnitude. Estimated maximum velocities during the movement reached 80 m/s. The runup is the highest recorded and on an empirical runup plot is highly anomalous in relation to the height of the descent slope. The case history illustrates the limitation of a dynamic model applied to a rock avalanche when it is assumed that the centre of gravity of the mass is displaced from the highest point on the detachment surface to the farthest tip of the debris. It also demonstrates that massive detachments have taken place in the Mackenzie Mountains in the comparatively recent past.
