The contribution of geology to debris slides on Vancouver Island, BC

Twenty six debris slides of approximately one hectare and larger were examined in the Tsitika and Schmidt Creek watersheds on northern Vancouver Island. Physical attributes were recorded for all sites. Bedrock and surficial geology played a significant role in the occurrence and distribution of these debris slides, both temporally and spatially. Bedrock consists of the Karmutsen Volcanics; an accumulation of submarine and subareal flood basalts, and of Island Intrusives; a granitic intrusive complex. Debris slides occurred preferentially over the Karmutsen Volcanics, and along dip slopes. Further, debris slides occurred preferentially in tills as opposed to any other surficial sediments. Debris typically initiated on steep upper-slopes, at the bedrock till interface. Further downslope they tended to continue sliding on a weathered-intact sediment interface and the role of bedrock was diminished.