ORIGIN OF THE BLUE HILLS FELSENMEER STATE NATURAL AREA #74, RUSK COUNTY, WISCONSIN

A felsenmeer exhibits angular boulders of uniform size resting on low-angle slopes. Felsenmeers may indicate intense freeze-thaw processes during periglacial conditions. The Blue Hills Felsenmeer State Natural Area in Rusk County, Wisconsin, was set aside to preserve an unusual boulder-covered valley. The aim of our study is to determine if this site is a true felsenmeer (rocks frost-shattered in place) or if it is a talus deposit (associated with falling rocks).

The valley at the Blue Hills site (NW1/4 Sec. 31, T35N, R9W; Strickland 7.5' quadrangle) is 25 m deep, 300 m long, and trends east-west. The Blue Hills site was not glaciated during the late Wisconsinan Glaciation. The valley walls are covered by angular quartzite boulders with an average diameter of 0.7 m. The boulders are derived from subhorizontal beds of the underlying Precambrian Barron quartzite. The quartzite beds are up to 0.5 m thick and vertical joints trend approximately north-south and east-west. Valley-floor elevations decrease approximately 18 m from the head of the valley to the mouth. The midpoint of the longitudinal profile displays a bulge up to 9 m above the adjacent valley floor. Typical valley-wall slopes are about 25 degrees. These slopes are much higher than reported for other felsenmeers (<10 degrees). A few quartzite outcrops form flat benches 5 m wide and tens of meters long parallel to the valley's long axis. These are present along the walls of the felsenmeer valley approximately two-thirds of the way up the slope. Block fields are present above and below the benches. The block fields are indented slightly below the bedrock benches. This might indicate the deflection of falling rocks around the bedrock benches and suggest a rock-fall (talus) origin for the boulders. Angular quartzite blocks on gently sloping uplands around the site might represent a true felsenmeer.

The steep slopes and indentations suggest a gravity-fall origin for the block fields within the valley and a feature that is a talus, and not a true felsenmeer. A ground-penetrating radar survey is planned for spring 2006 to determine the depth to bedrock below the boulders in the valley. If the feature is a talus, the boulders should be thicker at the base of the valley wall. Using these results it should be possible to determine the genesis of the site.