Wave Erosion of Brateng Geomorphic Surface in Lake Allison (Missoula Floods) and Deposition of Erratics, Mastodon Bones and the Malpass Stratigraphic Unit

Frank Reckendorf received his BS in Geology in 1961 from Iowa State University and then MS in Geology in 1963.

He received his Ph.D in Soil Science from Oren State University in 1973. He was 31 years employed as geologist for Soil Conservation Service. His firm, Reckendorf & Associates, was consulting for 30 years in Geologic Engineering

The last Missoula Flood in the Willamette Valley was one of the largest and occurred approximately 12,800 to 15,000 years ago. That flood formed Lake Allison which had a top elevation of 400 ft. above sea level based on lacial erratics. The pool of Lake Allison has an estimated maximum length of 111 miles long, average width of 31 miles, and average depth of 200 ft. A lake of at least this size would be necessary to generate the amount of wave erosion that produced the wave cut platform called the Brateng Geomorphic Surface. Sediments produced by this wave erosion formed the downstream Malpass Channel and sediments in Lake Allison.

 Soils on the Dolph Surface that were wave eroded had well developed Bt horizons. In some locations the wave erosions cut the land down to bedrock. Fractures at the top of the bedrock have been found to have red clay skins derived from the previous soil that was eroded off. In locations gullies formed on the side slope as the lake level dropped. As Lake Allison started to drain, side valley alluvium covered the eroded paleosol and preserved the clay skins on the fractures. The humas rich eroded soils drained down to the valley floor to form a clay rich sediment. As runoff channels developed they formed one or more high clay suspended load channels herein called Malpass channels. The valley at the time may have been braided with numerous Malpass Channels.