ERRATIC BEHAVIOR ON RATTLESNAKE MOUNTAIN, HANFORD REACH NATIONAL MONUMENT, SOUTH-CENTRAL WASHINGTON

Ice-rafted debris accumulated in slackwater areas up to an elevation of 1200 ft within the Pasco Basin during repeated Pleistocene cataclysmic floods. Floodwaters backed up behind a hydraulic constriction at Wallula Gap, forming temporary Lake Lewis and depositing ice-rafted erratics and bergmounds along the gently sloping flanks of Rattlesnake Mountain (RM). Ice-rafted debris is of three types: 1) isolated erratics, 2) erratic clusters, and 3) bergmounds. Bergmounds consist of accumulations of erratics, which display some topographic relief, usually in the form of low conical-shaped mounds.

A study of the ice-rafted debris is being performed in a long-protected, sparsely vegetated, 17 mi2 area on the NE flank of RM, now part of the Hanford Reach National Monument. Locations of erratics with >1 ft2 area (planview) and bergmounds are being recorded using a hand-held GPS. Additional information is being gathered on: 1) elevation, 2) lithology, 3) size, 4) roundness, 5) shape, and 6) surface characteristics of erratics. Greater than 95% consist of rock types other than indigenous basalt; >75% being of granitic composition. Other lithologies, in order of decreasing abundance, are diorite, quartzite, basalt, schist, gneiss, and argillite. Most erratics are either subrounded or rounded, followed by subangular; angular clasts are least common. The surfaces of most erratics are unweathered to moderately weathered. Approximately 30% of erratics, perhaps derived from older pre-Wisconsin floods, are strongly weathered.

The distribution of erratics is non-uniform and their overall size and frequency decreases with elevation. Decreases in the number of erratics with elevation are attributed to a greater number of less-than-maximum floods. Decreases in size may be due to either: 1) larger floods producing smaller icebergs, or 2) large icebergs, capable of rafting more and larger debris, becoming grounded well away from the ancient lakeshores. Since bergmounds are generally absent above 1000 ft, we prefer the later explanation. Erratics appear to concentrate along the SE sides of a series of NE-trending gullies within the study area. Variable flow velocities across this uneven surface may have created eddies, forcing icebergs to collect in the deeper and quieter waters along the back sides of these gullies.