INVESTIGATION OF THE OLYMPIA STRUCTURE AT VANCE CREEK, MASON COUNTY, WA
This study presents the results of ground-based, proton precession magnetometer surveys performed along forest roads concentrated around Vance Creek in Olympic National Forest. The field area was selected based on findings implying that faulting may have been responsible for shaping the region's present-day lithology and topography, and a magnetic anomaly appearing in the aforementioned aeromagnetic survey. Magnetic data show local variations in intensity on the order of 1,000 nT within the same geologic unit. These variations occur in multiple closely-spaced roughly north-south transects. Further, these magnetic variations occur at extensions of the locally east-west striking Vance Creek valley. The geometry and nature of the magnetic variations serve to further support the hypothesis that the a subsurface geologic structure, such as the Olympia Structure, extends through the region and is locally responsible for shaping the Vance Creek valley.