Student Abstracts: General Sciences at PNNL

Erratic Behavior on Rattlesnake Mountain, Hanford Reach National Monument, South-Central Washington. ELYSIA JENNETT (Northern Arizona University, Flagstaff, AZ 86011) BRUCE BJORNSTAD (Pacific Northwest National Laboratory, Richland, WA 99352).

A study of the ice-rafted debris has been performed in a long-protected, sparsely vegetated, 17 mi2 area on the NE flank of Rattlesnake Mountain, now part of the Hanford Reach National Monument. The objective of this study was to catalog ice-rafted debris on Rattlesnake Mountain and look for trends in their distribution. Ice-rafted debris is of three types: 1) isolated erratics, 2) erratic clusters, and 3) bergmounds. Locations of erratics with >1 ft2 area (planview) and bergmounds were recorded using a hand-held GPS. Additional information was 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. 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 can be explained by an increasing number of smaller 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 N 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 backsides of these gullies.