Description: Distinctive mounds of till occur 100 to 240 meters above the present Columbia River along the western margins of the Pasco and Quincy basins. These mounds were formed by melting of grounded icebergs after inundation of the basins by the catastrophic glacial Lake Missoula flood(s). Most of the bergmounds are circular in plan and range in size from small clusters of erratics to mounds 50 meters in diameter and up to 4 meters high. The detritus is composed predominantly of granite, slate, argillite, quartzite, gneiss, and basalt, with grain size ranging from clay-size paticles to boulders up to 3 meters intermediate diameter. The pebbles and cobbles are glacially polished and fine-grained clasts are often striated. The bergmounds can be grouped into 4 classes on the basis of clast lithology. The most commonly occurring bergmounds are composed of 85 to 100 percent granitic debris. Bergmounds exhibiting mixed lithologies are also common. Less common are bergmounds composed of greater than 85 percent basalt and mounds composed predominantly of quartzite. The bergmounds occur in groups and are rarely found as isolated mounds. The elevation and distribution of bergmounds are related to fluvial currents and depths of the flood waters with iceberg grounding in regions of slack water and/or eddy currents.
Date: October 1, 1978
Creator: Fecht, K.R. & Tallman, A.M.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department