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The Texas Creek rock avalanche is a prehistoric deposit in the Fraser River Canyon, 17 km south of Lillooet, southwestern British Columbia, Canada. Original mapping suggested that the debris consisted of two landslides: a 45 Mm3 event deposited after the Mazama tephra but before about 2 ka ago, and a 7.2 Mm3 event about 1.1 ka ago. The proposed timing of the younger landslide was correlated with a decline in the First Nations population and was proposed as an agent of cultural collapse driven by its impact on salmon returns vital to the population's sustenance. We provide six surface exposure ages using 10Be from boulder tops, with three samples from each surface that were originally posited to be older and younger debris. The six samples yielded similar ages suggesting the landslide deposit represents a single event with an average age of 2.28 ± 0.19 (2σ external error) ka before 1950 AD. Evidently, the landslide played no role in the cultural collapse. Fraser River Holocene incision rates, estimated pre- and post-landslide are between 13 and 24 mm/yr, consistent with previous estimates for the mid-Fraser River region. Landslide timing is coincident with the explosive eruption of Mount Meager, 120 km to the northwest, and with a possible landslide at Mystery Creek 85 km to the west and 65 km south of Mount Meager. The landslide may have been seismically triggered, but attribution is speculative. [ABSTRACT FROM AUTHOR]

Rock slides quite commonly transform into flow-like landslides along their runout paths. At three initial rock slides in northern British Columbia, which occurred between 2002 and 2005, around 50–70% of the entire runout distance is composed of debris avalanche and debris flow deposits, which is comparable to other composite landslides around the world. Saturated ground conditions at the time of rock sliding make entrainment or undrained loading as agents of flow transformation from initially dry rock slides to partially saturated debris avalanches, respectively, fully saturated debris flows likely. Hummocks occur as clusters within the rock slide parts, whereas the flow-like depositions have more subdued morphologies. We show that late- or post-emplacement motion of individual hummocks is possible and can even divert in direction from the dominant landslide trajectory by responding to the underlying topographic gradient. Analyses of the well-preserved deposits suggest hummock formation above a basal sliding plane (low-angle normal fault) and along subordinate shear zones (high-angle normal faults) within a largely translational mass movement, thereby supporting the hypothesis of hummock formation proposed by Paguican et al. (2014). Pebbles on top of rock slide-debris avalanche boulders and on top of snapped-off trees record much higher dynamic debris flow heights. These and other features are not recorded on ancient landslides due to rapid erosion. [ABSTRACT FROM AUTHOR]