SEDIMENTOLOGY OF THE BLUE HILLS FELSENMEER STATE NATURAL AREA, WISCONSIN

The Blue Hills Felsenmeer valley in Rusk County, Wisconsin, trends east-west, is 25 m deep, and is 300 m long. The valley walls slope at 25° and are strewn with angular Barron Quartzite boulders. Steep slopes suggest a rock fall (talus) origin for the block field rather than freeze-thaw processes acting in situ (the process of felsenmeer formation). The purpose of this study is to examine the sedimentology of the block field and assess whether it is a felsenmeer or talus deposit.

The sedimentology of the Felsenmeer was studied during ten field days. Nine grain-size distributions (n = 306 to 520) were determined by tossing a rope over the blocks and measuring long-axis rock diameters in contact with the rope. Each sample area was mapped using a GPS unit. A ground penetrating radar (GPR) survey was conducted along the long axis of the Felsenmeer valley to discern any internal block-field stratigraphy. Software package SPSS (v. 16) was used to plot grain-size histograms.

Median clast diameters are larger near the valley floor. Higher elevation zones have median clast sizes of 25cm and 35cm compared with areas directly below exhibiting median clast sizes of 35cm and 45cm, respectively. The southern wall has larger, more tabular clasts (45 cm median diameters). These display rippled surfaces, possibly due to breakage along bedding planes dipping gently to the north (~20°). Clasts on the northern wall are more blocky and exhibit fewer ripple marks. GPR data reveal semi-continuous westward dipping reflections within the block field, and also a lower continous reflectionbeneath the block field.

Sediment sorting within the Felsenmeer suggests that the Blue Hills Felsenmeer is not in situ material and is not a true felsenmeer. The sorting and steep block-field slopes indicate a talus deposit.