Valley Deposits Immediately East of the Channeled Scabland of Washington. I

Abstract

Forty-one valleys whose drainage enters the eastern margin of scabland spillways are known to contain deposits made by glacial waters. Thirty-nine of these valleys never have carried drainage from glaciated areas, and the glacial waters in them must have been backed up from the scabland. These deposits possess such extraordinary features and relations that they, like the scabland topographic forms, are considered unique. 1. Each separate area of backwater is recorded by a widespread mantle of silt containing abundant grains of unweathered basalt (the country rock) and pebbles of foreign rock. 2. In each, this mantle extends up to a definite upper limit on the valley slopes and along the valley lengths. 3. These upper limits in each case agree closely with the upper limit of scabland where the valley enters. 4. The altitudes of the upper limits, constant throughout any one backwater area, vary with different pondings, forming a descending series from north to south along the scabland gradient. 5. Remarkabl...