| Since the end of the Fraser alpine glaciation about 10,000 years ago, three significant lahar-producing eruptive periods have occurred at Mount Hood. The lahars were confined to river basins originating on the southwest flank of the mountain. The older, timberline eruptive period occurred 1400 to 1800 year B.P. (Crandell, 1980). Deposits form the Timberline eruptive period (informally termed Timberline-age in this report) can be traced the length of the ZigZag and Sandy Rivers to the Columbia River. Flows from the Zigzag eruptive period (informally termed Zigzag-age in this report) have been identified only in the middle Zigzag basin and in the upper Sandy basin at Old Maids Flat. Deposits from the Old Maid eruptive period (informally termed Old Maid-age in this report) have been identified from the upper Sandy basin downstream to 2.5 km below the confluence of the Sandy and Salmon Rivers near the hamlet of Brightwood and at a single locality below the confluence of the Zigzag and Little Zigzag Rivers. -- Cameron and Pringle, 1986 |
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