GPR SURVEY OF THE BLUE HILLS FELSENMEER, WISCONSIN: PROFILE OF A RELICT TALUS SLOPE

The Blue Hills Felsenmeer State Natural Area in Rusk County, Wisconsin, is a unique, 25 m deep valley completely lined with angular Barron Quartzite boulders. The valley is 300 m long, trends east-west, and displays a convex longitudinal profile with two crests at the midpoint rising up to 9 m above the adjacent valley floor. The purpose of this study was to determine if the valley is a felsenmeer (bedrock shattered in place) or a talus (rock fall deposit), as proposed by Thompson and Syverson (2006) and Hinke and Wittkop (2007).

A ground-penetrating radar (GPR) survey was conducted to determine the valley origin and reveal if the convex longitudinal profile is caused by a thicker accumulation of boulders near the valley midpoint (Thompson and Syverson, 2006). A backpack-mounted pulseEKKO 100 GPR unit was used to collect reflection data. To maximize depth of penetration, a 100 MHz transmitter and receiver antennae with 1 m separation were moved along a plastic surveyor's rope on the valley floor (Jol and Bristow, 2003). To provide horizontal resolution, traces were collected every 0.5 m. The laptop computer allowed for real-time observation of the reflection profile. Laser level elevation data was also collected and the reflection profile was corrected for topography in the lab.

The GPR survey shows a continuous, westerly dipping reflection between the open-work boulders and underlying intact quartzite. Boulder depths above this contact range from 1 to 9 meters. The thickest accumulation coincides with the valley crest near the valley midpoint. The GPR reflection profile provides conclusive evidence that at least part of the Blue Hills Felsenmeer is a talus deposit. Glacial meltwater flowing to the west eroded the box-canyon valley. As rocks fell onto the valley floor, the greatest thickness accumulated in the narrowest part of the valley and formed the crest near the valley midpoint.

This study was funded by a North-Central GSA Undergraduate Research Grant and a UW-Eau Claire Summer Research Experiences for Undergraduates Grant.