1

Sedimentology of the Blue Hills Felsenmeer

State Natural Area, Wisconsin

Orr1, I.M., Mohr1, A.R., Syverson1, K.M., and Jol2, H.M.

Department of Geology1

and Department of Geography2

University of Wisconsin, Eau Claire, WI 54702

(E-mail: syverskm@uwec.edu)

May 15, 2009

Information in this report is modified from a poster presented at the North-Central Geological

Society of America meeting in Rockford, IL, on April 3rd

, 2009. Official presentation reference:

Orr, I.M., Mohr, A.R., Syverson, K.M., and Jol, H.M., 2009, Sedimentology of the Blue Hills Felsenmeer

State Natural Area, Wisconsin: Geological Society of America Abstracts with Programs, v. 41,

no. 4, p. 63.

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Abstract (Orr et al., 2009)

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 reflection beneath 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.

Introduction

Felsenmeers are angular block fields resting on low angle slopes, and generally are the

result of freeze-thaw processes (Washburn, 1979: p. 219). The Blue Hills Felsenmeer State

Natural Area is located in Rusk County, Wisconsin (Fig. 1). The Felsenmeer is in an

unvegetated valley ~300 meters long and ~25 meters deep (Fig. 2, Thompson and Syverson,

2006). The bedrock and openwork boulders covering the valley are part of the Barron quartzite, a

tan to purplish rock correlative to the Baraboo quartzite (Campbell, 1986; Medaris and Dott,

2001). Mohr et al. (2009) have used ground-penetrating radar [GPR] to show that the

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“Felesenmeer” is actually a talus (a rockfall deposit).The goal of this study is to determine if

boulder sedimentology and associated landforms support a mass wasting origin for the Blue Hills

Felsenmeer. If significant clast sorting has occurred within the block field, this would indicate

boulder transport and a mass wasting origin (Ballantyne and Eckford, 1984; Wilson, 1988).

Clast size coarsens downslope on talus surfaces because of large, moving rocks have more

momentum than smaller rocks (Wilson, 1988; Dorren, 2003). In addition, protalus ramparts

commonly are associated with rockfall deposits at the base of cliffs (Fig. 3, Wilson, 1988;

Shakesby, 1997). This research is intended to supply the Wisconsin DNR with quality geological

interpretative materials for the Blue Hills Felsenmeer State Natural Area.

Figure 1. Location of the Blue Hills Felsenmeer State Natural Area, Rusk County,

Wisconsin. Modified from Hinke and Wittkop (2007).

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Figure 2. Photograph of the Blue Hills Felsenmeer valley looking to the west. The openwork

texture of the angular quartzite boulder field precludes the growth of vegetation (with the

exception of the small birch tree).

Figure 3. Protalus rampart formation. A. Snow accumulates at the base of cliff and falling rocks

bounce/slide across the snow field and accumulate in a pile away from the base of the cliff. B. When the

snow melts, the rocks at the end of the former snow field remain as a ridge (a protalus rampart).

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Methods

• Spent ten field days studying the sedimentology of the Blue Hills Felsenmeer during the

summer of 2008.

• Performed nine random grain size counts in different parts of the Felsenmeer valley. A

weighted string was thrown over the block field. Long axes of clasts in contact with the rope

were measured with a meter stick and tabulated in 10 cm bins.

• When possible, 500 clast diameters were measured to ensure a representative sample.

Sample area boundaries were defined by permanent physical features in the felsenmeer

valley and mapped using a Trimble Differential GPS.

• Made grain-size histograms and calculated statistics using SPSS. Independent variable T-

tests were run to compare sample means and discern whether the means are significantly

different at a 95% confidence interval.

• Used Trimble Differential GPS to record locations of protalus ramparts (Fig. 3) in the valley.

• Performed a GPR survey along 220 meters of valley floor to look for reflectors within the

block field suggesting internal sorting (and a rockfall origin).

• Used a backpack-mounted pulseEKKO 100 GPR system to collect real-time data. Used a

low-frequency (100 mHz) signal with an antennae separation of 1 m and step size of 0.5 m to

collect data with sufficient density to resolve steeply dipping reflectors.

Results

• Modal grain-size diameters are between 25cm and 45cm within the block field (Figs. 4, 5).

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Figure 4A & B. Grain-size distributions for GS6 (A, top) and GS7 (B, bottom), show

downslope coarsening (sorting). A T-test showed the means to be significantly different at a

95% confidence interval (T(911) = -18.812).

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Figure 4C & D. Grain-size distributions for GS9 (C, top) and GS8 (D, bottom), show downslope

coarsening (sorting). A T-test showed the means to be significantly different at a 95% confidence

interval (T(857) = -13.008).

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Figure 4E & F. Grain-size distributions for GS3 (E, top) and GS2 (F, bottom), show downslope

coarsening (sorting). A T-test showed the means to be significantly different at a 95% confidence

interval (T(710) = -13.186).

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Figure 4G, H, & I .

Grain-size distributions

for GS4 (G) and GS1 (H)

on the north wall and GS5

(I) on the south wall.

Median clasts sizes are

smaller on the northern

wall (G, H) than on the

southern wall (I).

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• Boulders on the southern valley wall are elongate, tabular, and commonly preserve ripple

marks (Fig. 5). The boulders also appear to have slid over the top of each other. Dorren

(2003) suggested that long tabular clasts are preserved because they do not fall down a slope,

but rather slide over the top of one another. The gentle northerly dip of the Barron Quartzite

at the field site (14-18 degrees, Hinke and Wittkop, 2007) may facilitate this process at the

Felsenmeer.

• Two protalus ramparts are present where the valley widens to the east and west (Cahow, n.d.,

Figs. 3, 6). These protalus ramparts located at the foot of steeply dipping block fields are

more evidence for rock falls in this area.

• The GPR profile reveals multiple, poorly developed reflector layers on the west side of the

bulges near the valley midpoint (Fig. 7). This is more evidence of sorting and a rockfall

origin for the block field.

• The talus at the site probably formed during periglacial conditions following the late

Chippewa Phase of the Wisconsin Glaciation (15,000-18,000 yrs ago, Clayton et al., 2001;

Syverson, 2007). Ice during the late Chippewa Phase was sufficiently high to supply

meltwater to the Felsenmeer valley (Hoaglund et al., 2007). A pre-existing block field would

have filtered out rounded erratic boulders on the proximal (east) side of the block field. The

absence of such erratic boulders suggests the block field formed during enhanced freeze-thaw

and rock-fall conditions post-dating the late Chippewa Phase (Clayton et al., 2001).

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Figure 5. Cross

sectional profiles

across the Felsenmeer

valley (vertical

exaggeration 3x, see

Figs. 6B and 7 for

locations).

5A. Easternmost cross

section. Grain-size

count areas (GS*) are

indicated, and sediment

coarsens downward

(Fig. 4E,F,I). Tabular

clasts on southern wall

display ripple marks.

5B. Valley floor crest. GPR profile indicates a

thicker accumulation of

boulders in this area

relative to other areas.

Grain-size counts (GS*,

Fig. 4C,D) show

downward coarsening.

5C. Cross section to

the west where the

valley widens. The

GPR profile shows a

thin boulder

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Figure 6. Location maps on LiDAR base image from Hinke and Wittkop (2007). A. Grain-

size count areas. Green lines mark the locations of protalus ramparts. B. Valley cross

section locations for Fig. 5. Grain-size count areas are dashed for reference.

Conclusions

• Clasts at the Blue Hills Felsenmeer coarsen downslope, consistent with talus observations

elsewhere (Dorren, 2003). This sorting suggests the Blue Hills block field is a talus (rockfall

deposit) rather than a true, in-situ felsenmeer.

• Protalus ramparts indicate rock-fall processes acting at the time of block-field formation.

• GPR profile shows poorly defined internal stratigraphy and sorting resulting from gravity

sorting.

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• The lack of glacial erratics in the valley suggests the block field formed after the late

Chippewa Phase ice maximum during the late Wisconsin Glaciation (15,000-18,000 years

ago).

Acknowledgments

• Dr. April Bleske-Rechek, UWEC Dept. of Psychology, for statistical assistance.

• Dr. Sean Hartnett, UWEC Dept. of Geography, for assistance with GPS equipment and

lab work.

• Rusk County Parks and Forestry Department for access to the site and logistical support.

• UW-Eau Claire Office of Research and Sponsored Programs for funding the study.

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15

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