SCOURING ON BURIED BARABOO QUARTZITE SURFACES: IMPLICATIONS FOR DEVILS LAKE STATE PARK, WI, POTHOLES

Recent exposures of Baraboo Quartzite along the retreating highwall in Williams Quarry contained large scour features, potholes, and channels buried beneath Cambrian conglomerate. An outcrop of scoured and potholed quartzite with Cambrian sandstone solidly attached to scoured surfaces exists near the east nose of the Baraboo syncline. A similar exposure was recently discovered ~2 mi to the southwest. These exposures renewed interest in the age and origin of the potholes long known at Devils Lake State Park. Although the age of the Windrow Formation is controversial (D to K), some indicate that the sand and gravel of the East Bluff Member were the grinders for the Devils Lake potholes during the Pleistocene.

In fall 2007, excavating began near the Potholes Trail. The first pothole unearthed is at the base of the known potholes. The top measures about ~1.5 m across. A 0.3 m furrow coils to the bottom and connects to another pothole under the trail steps. Cobbles and boulders of mostly quartzite and some chert, many subrounded to rounded, occupied the hole along with stratified sand and gravel, presumably the East Bluff Member of the Windrow. The second excavated pothole, located about 9 m further down the trail, is cylindrical, ~1.5 m deep, and contained only topsoil fill and small boulders and cobbles of quartzite and chert, some subrounded to rounded. The third exposure, 3 m east of the trail, reveals a complexly scoured quartzite surface about 2 m down with this sequence of overlying sediments: angular quartzite talus with topsoil fill (0 – 0.5m down), gray silt with scattered angular quartzite pieces (0.5 – 0.75 m), stratified sand and gravel, presumably the East Bluff Member (0.75 – 1.5 m), mat of oak leaves (1 cm; C14 date pending), stratified East Bluff Member (1.5 – 2m). Contained within the overlying sediment were two large angular quartzite boulders, small boulders of quartzite and occasional chert, some rounded, and a large, well-rounded quartzite boulder with percussion marks similar to ones found in Williams Quarry.

Because much of the stratified Windrow sand and gravel laid above a mat of well-preserved leaves and well above the scoured surface, it is suggested here that the Windrow sediments were fairly recent fill and not the grinders of the potholes. Thus, the potholes are older than Pleistocene in age.