Abstract.A large rock avalanche occurred at 03:27:30 PDT,

Three retrogressive thaw slumps of varying age have been initiated by erosion of ice-rich glaciolacustrine sediments on a bend of Stewart River, 3 km upstream from Mayo, Yukon Territory. Two of the slumps are presently active; the third stabilized before 1944. The rate of retreat of the active slump headwalls between 1949 and 1987, determined from aerial photographs and ground surveys, is up to 16 m.yr". Floors of the active thaw slumps contain well-defined vegetation successional communities that are distinct from the local, mature boreal forest. Although a few clumps of mature forest vegetation survive the fall into the slump, a birch/white spruce sere, similar to the original forest, is re-established after a period of 35-50 years. Changes in soil calcium carbonate and soil structure profiles on disturbed surfaces of varying age demonstrate the initiation of pedogenesis in the floor of the stabilized slump, but assays of pH, organic carbon and total nitrogen indicate that after about 40 years the new soils remain immature. Comparison of ground temperatures in the stabilized thaw slump and at undisturbed sites in the area indicates that the ground thermal regime may return to local conditions a century or more after disturbance.

Mount Meager massif, the northernmost volcano of the Cascade volcanic belt, has been the site of very large (>107 m3) landslides in the Holocene Epoch. We document two complex landslides at Pylon Peak, one of the peaks of the Mount Meager massif, about 7900 14C and 3900 14C years ago (about 8700 and 4400 calendar years ago). Together, the two landslides displaced ~ 6 × 108 m3 of volcanic rock from the south flank of Pylon Peak into nearby Meager Creek valley. Each landslide consisted of at least two phases, an early debris flow resulting from failure of hydrothermally altered pyroclastic rock at mid levels on the mountain and a later rock avalanche from a higher source. Both debris flows likely traveled down Meager Creek, and preliminary evidence from drilling indicates the 4400-year-old event traveled down Lillooet River into areas that are now settled and where population density is increasing rapidly. The mobility of the debris flows was due to the high content of fine, weathered volcanic sediment and the availability of sufficient water. The causes of the landslides are a wet climate and the presence of weak, hydrothermally altered volcanic rock containing abundant phreatic water on glacially oversteepened slopes. The landslides may have been triggered by earthquakes or by upwelling of magma to shallow depths within the volcano. However, they may also have occurred without specific triggers following extended periods of progressive weakening of the volcanic rocks.

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