Origin of the Blue Hills State Natural Area #74, Rusk County, Wisconsin by Andrew H. Thompson and Kent M. Syverson Department of Geology, University of Wisconsin, Eau Claire, WI 54702 (E-mail: syverskm@uwec.edu) Information in this report is modified from a poster presented at the North-Central Geological Society of America meeting in Akron, OH, on April 15 th, 2006. Official presentation reference: Thompson, A.H., and Syverson, K.M., 2006, Origin of the Blue Hills Felsenmeer State Natural Area #74, Rusk County, Wisconsin: Geological Society of America Abstracts with Programs, v. 38, no. 4, p. 25.
Abstract (from Thompson and Syverson, 2006) 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. Introduction A felsenmeer (German for sea of rocks ) is a feature that exhibits fairly uniformly sized, angular rocks resting on a low-angle slope (Washburn, 1973:191). Felsenmeers are generally caused by intense freeze-thaw processes in an area that shatter existing bedrock. The Blue Hills Felsenmeer State Natural Area (Fig. 1) is defined by two valley walls strewn with angular boulders that meet for approximately 300 meters (Fig. 2; Cahow, n.d.). Page 1
Figure 1A. Location of the Blue Hills State Natural Area. Extent of the Barron quartzite is from Ostrom (1995). Figure 1B. Topography surrounding the east-west-trending Blue Hills Felsenmeer valley. From Strickland USGS 7.5' Quadrangle, Wisconsin. The bedrock in the Blue Hills and blocks in the felsenmeer are made of the Barron quartzite, a rock approximately 1.63 to 1.75 billion years old (Fig. 1A; Holm and others, 1998). The angular quartzite blocks in the valley walls are up to 0.7 meters in diameter and generally tan to grey in color (Fig. 2). The Blue Hills were entirely covered by glacial ice during the early Chippewa Phase of the late Wisconsinan Glaciation (Johnson, 1986; Syverson and Colgan, Page 2
2004), and the valley might have been eroded at that time. Portions of the Blue Hills were subsequently covered during the late Chippewa Phase, but it is uncertain if the Blue Hills ice margin was sufficiently high to supply water for valley erosion. Cahow (n.d.) theorizes that the Blue Hills Felsenmeer valley was eroded initially by a normal stream, and then glacial meltwater quickly deepened the valley 15,000 to 20,000 years ago. After the valley was abandoned, periglacial conditions enhanced frost-wedging and shattered the quartzite seen in the valley today. The aim of this study is to determine if the site is a true felsenmeer (rocks frost-shattered in place) or if it is talus (a gravity-fall deposit, Fig. 3). If it is a true felsenmeer, this could be further evidence of permafrost conditions that existed in the area during the late Wisconsinan Glaciation from about 25,000-10,000 years ago (Clayton and others, 2001). Our research is also intended to supply the WDNR with quality geological interpretive materials for the Blue Hills Felsenmeer State Natural Area. Figure 2. Picture looking to the west down the Blue Hills Felsenmeer valley. Page 3
Methods Worked in the field for five days during the summer and fall of 2005. Performed reconnaissance investigation of uplands surrounding the felsenmeer valley. Measured elevation of valley floor and adjacent channel with a surveying rod, Brunton compass, and steel chain (Fig. 2). Measured orientation of joints and bedding in quartzite outcrops. Compared morphology of boulder-strewn valley walls to exposed bedrock outcrops. Analyzed and plotted elevation data using Microsoft Excel to produce a longitudinal profile of the felsenmeer valley. Results The slopes of the Blue Hills Felsenmeer valley walls are significantly higher (25 degrees, Fig. 3) than for other felsenmeers that more typically slope approximately 10 degrees (Davis, 2001:164). Angular blocks imply that water or glaciers did not transport the boulders in the felsenmeer (Fig. 4B). The crest in the valley floor near the center of the valley profile likely represents more the significant accumulation of talus (Fig. 5). If that is not true, then the valley needed to form beneath the glacier in order to allow water to flow uphill under hydraulic pressure. The tributary channel (Fig. 6) slopes upward toward the east and heads at a flat plain going uphill. This suggests a meltwater origin for the valley. Indentations in the block field on the north valley wall are located below bench-like bedrock outcrops (Fig. 6). These bedrock "benches" are present in the block field itself (Figs. 8, 9). Gently sloping uplands north of the Blue Hills Felsenmeer valley display angular quartzite boulders protruding from the forest litter. These boulders did not fall onto these uplands. Page 4
The thickness of the boulder accumulation on the uplands is unclear. These boulders might represent a true felsenmeer similar to those described elsewhere (Sugden and Watts, 1977). Figure 3. Formation of talus and felsenmeer. Both methods of formation are driven by mechanical weathering (breakage) of the rock -- something that would be accelerated by freeze-thaw processes acting under periglacial conditions. The primary difference is that talus blocks move downslope by gravity (Fig. 3B), and then accumulate in a thick deposit (talus) at the base of the cliff (Fig. 3C). Felsenmeer blocks form as rocks shatter during numerous freeze-thaw cycles, but the blocks remain in place (Figs. 3D, 3E). Thus, a felsenmeer covers the entire landscape with a relatively uniform thickness of angular blocks (Fig. 3F). Page 5
Figure 4A. Bedrock outcrop on the north side of the valley. Horizontal bedding and near-vertical joints are visible. Joints make freeze-thaw processes more effective. Figure 4B. Angular quartzite boulder near the valley floor. The size and shape of the boulder is typical to others in the block field. The angular shape implies breakage of the rock without transport by water or glacial ice. Page 6
Figure 5. Longitudinal profile of the Blue Hills Felsenmeer valley floor. The valley-floor profile is convex with a crest rising 9 m (30 ft) above the valley floor to the east. If this profile represents the original valley floor (yellow outline), then the valley had to be incised subglacially because surficial water cannot flow uphill. However, other data suggests that this crest probably formed as rock falls filled more of the valley in this area and obscured the original valley floor (red outline). Figure 6. Map of the Blue Hills Felsenmeer valley with only the block field on the north valley wall represented graphically. All indentations in the valley wall are below bedrock outcrops. The tributary channel to the east is generally dry, slopes into the main valley, and heads at a flat surface to the east. This tributary channel must have been fed by glacial meltwater that helped incise the felsenmeer valley. Certainly ice of the early Chippewa Phase reached this elevation, but it is unclear if ice of the late Chippewa Phase could have supplied water to this tributary channel. Page 7
Figure 7. Looking west from midway up the southern talus slope. Photo taken near the crest in the central part of the longitudinal profile (Fig. 5). A bedrock bench is hidden in the area of the X" (Fig. 8). The depth of the valley in the foreground is approximately 25 m (80 ft). Figure 8. Photo of bedrock "bench." The flat bench surface is controlled by subhorizontal bedding in the quartzite bedrock. To the left ("Y") is a steep block field at the head of the bench. Just off the picture to the right ("Z") is the head of the main block field extending down to the valley floor. This bench is approximately two-thirds of the way up the north valley wall. Figure 9 shows how this "bench" may have influenced block field morphology. Page 8
Figure 9. Falling rocks accumulate on the bedrock bench from steeply sloping area. These blocks do not reach the felsenmeer valley floor to the right. Thus, the valley wall is indented slightly below the bedrock bench (Fig. 6). Conclusions and Future Work The steeply sloping block fields in the valley, the convex longitudinal valley profile, and the indentations below some bedrock benches are all indicative of a talus origin for the felsenmeer. The angular boulders on the rounded uplands north of the felsenmeer valley are likely a true felsenmeer. A GPR survey of the felsenmeer valley is planned for summer 2006 to determine if the boulder thickness is uniform (thus implying a felsenmeer) or if boulder thickness is greater at the base of the slope (implying a gravity-fall origin for the blocks). Further work will be done during summer/fall 2006 to study the glacial history of the Blue Hills region. The maximum extent of the Chippewa Lobe during the late Chippewa Phase will be mapped to determine if meltwater of this phase could have eroded the valley now containing block fields. Page 9
Acknowledgments Adam Cahow, Professor Emeritus - UWEC Geography (For his guidance and assistance in the field) Wayne Tappon, DNR Liaison Forester to the Rusk County Forest (For his advice in the early stages of planning for the research) Paul Teska, Rusk County Forest Administrator (For his help throughout the project in getting access to the site) UW-Eau Claire Center of Excellence for Faculty and Undergraduate Student Research Collaboration (For funding this study) Bibliography Cahow, A.C., no date, The Blue Hills Felsenmeer State Natural Area: Unpublished manuscript available from the Wisconsin DNR Bureau of Endangered Resources office, 7 p. Clayton, L., Attig, J.W., and Mickelson, D.M., 2001, Effects of late Pleistocene permafrost on the landscape of Wisconsin, USA: Boreas, v. 30, no. 3, p. 173-188. Davis, N., 2001, Permafrost: A guide to frozen ground in transition: Fairbanks, University of Alaska Press, 351 p. Holm, D., Schneider, D., and Coath, C.D., 1998, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia: Geology, v. 26, no. 10, p. 907-910. Johnson, M.D., 1986, Pleistocene geology of Barron County, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 55, 42 p. Ostrom M.E., 1995, Bedrock Geology of Wisconsin: Wisconsin Geological and Natural History Survey Sugden, D.E., and Watts, S.H., 1977, Tors, felsenmeer, and glaciation in northern Cumberland Peninsula, Baffin Island: Canadian Journal of Earth Science, v. 14, p. 2817-2823. Syverson, K.M. and Colgan, P.M., 2004, The Quaternary of Wisconsin: a review of stratigraphy and glaciation history: Quaternary Glaciations - Extent and Chronology, Part II, p. 295-311. Thompson, A.H., and Syverson, K.M., 2006, Origin of the Blue Hills Felsenmeer State Natural Area #74, Rusk County, Wisconsin: Geological Society of America Abstracts with Programs, v. 38, no. 4, p. 25. Washburn, A.L., 1973, Periglacial processes and environments: New York, St. Martin's Press, 320 p. Page 10