ORIGIN OF THE CONVEX LONGITUDINAL VALLEY PROFILE, BLUE HILLS FELSENMEER STATE NATURAL AREA, RUSK COUNTY, WISCONSIN

A felsenmeer is a “sea” of angular boulders that has formed in situ on a gently sloping surface. The Blue Hills Felsenmeer State Natural Area valley (NW1/4 Sec. 31, T35N, R9W; Strickland 7.5' quadrangle) is 300 m long, 25 m deep, and has walls sloping at approximately 25 degrees. The felsenmeer valley has a convex longitudinal profile marked by angular quartzite boulders. A topographic map (CI=2 ft) generated using Barron County LiDAR data shows that the midpoint “bulge” rises up to 30 ft (9 m) above the adjacent valley floor.

The purpose of our study is to determine what has caused the convex longitudinal valley profile. Thompson and Syverson (2006) suggest that the convex profile was caused by falling rocks. A boulder accumulation that is much thicker beneath the bulge than the adjacent parts of the valley floor would support a talus (rock fall) origin. A uniform boulder thickness over the intact bedrock surface would suggest in situ frost-shattering that has occurred over a convex valley floor.

We will use Ground Penetrating Radar [GPR] in the felsenmeer valley to determine the thickness of boulders and the shape of the underlying bedrock surface. GPR works by sending a radio wave into the ground. The wave bounces back to the surface when it hits a material with a different velocity. The two-way travel time and wave velocity are used to determine the depth of the reflection horizon. We will use a frequency that provides the proper resolution and depth of penetration (50 or 100MHz). The GPR survey and the detailed land-surface topographic map will permit us to document the thickness of boulders and the longitudinal profile of the intact bedrock surface.

GPR results will help us understand the origin of Blue Hills Felsenmeer valley. Last fall our initial attempt to perform the field GPR survey was stymied by instrumentation problems. However, if the future GPR results indicate that boulders are much thicker beneath the bulge at the midpoint of the valley, then at least that portion of the Blue Hills Felsenmeer is a talus. Uniform boulder thicknesses along the valley floor would reinforce a felsenmeer origin for the boulder field. If the longitudinal profile of the valley-floor bedrock surface is convex, then pressurized subglacial meltwater must have incised the original valley floor prior to the late Chippewa Phase of the Wisconsin Glaciation.