POTENTIAL MELTWATER INCISION OF THE BLUE HILLS FELSENMEER VALLEY, RUSK COUNTY, WISCONSIN, DURING THE LATE WISCONSIN GLACIATION

The Blue Hills Felsenmeer State Natural area is an unusual valley of angular quartzite boulders in a high-relief area of Rusk County, WI (Thompson and Syverson, 2006). The valley at the Blue Hills site (NW1/4 Sec. 31, T35N, R9W; Strickland 7.5' quadrangle) is 25 m deep and 300 m long. The valley has a small modern water-catchment area that heads at an elevation of 1456 ft (444 m) based on LiDAR data. Cahow (no date) proposed the valley was cut by Chippewa Lobe meltwater during the late Chippewa Phase of the late Wisconsin Glaciation. The purpose of this study is to determine if the Chippewa Lobe ice surface was sufficiently high during the late Chippewa Phase to supply meltwater to erode the valley.

In order to do this, we mapped the maximum extent of the Chippewa Moraine. We used domestic well logs, the Rusk County soil survey, aerial photographs (1:16,000 scale), and 7.5' topographic maps to construct a preliminary 1:24,000-scale glacial sediment/landform map. Ten field days were spent studying the proposed glacial sediment/landform contacts in the felsenmeer area. Work included excavating slumped road-cuts, describing exposed sediment outcrops, and digging bore holes to examine glacial sediment. Field observations verified the presence of chaotic hummocks, kettles, and ice-walled-lake plains in the Chippewa Moraine. The outermost extent of the Chippewa Moraine provides reasonable minimum estimate of ice extent during the late Chippewa Phase.

The method of Clark (1992) was used to determine minimum ice-surface elevations near the head of the felsenmeer valley. This method requires measuring hummock-crest elevations within 2 km of the former ice margin. This provides a minimum ice-surface elevation estimated by Clark (1992) to be within 30-100 ft (10-30 m) of the actual value.

We measured a hummock crest elevation between 1400-1410 ft (427-430 m). The hummock, located 1.5 km south of the felsenmeer valley head, puts glacier ice within 56 ft (17 m) of the valley head threshold. This is well within the 30-100 ft (10-30 m) range. It is therefore possible that glacier ice from the late Chippewa Phase of the late Wisconsin Glaciation could have provided meltwater to erode the valley. It is also possible that an earlier event (such as the early Chippewa Phase or a pre-Wisconsin event) could have supplied meltwater to erode the valley.