DOCUMENTATION OF A UNIQUE OUTCROP, BARABOO AREA, WISCONSIN

Three unique features of a recently discovered outcrop in a quarry between Baraboo and Portage are: 1) a stream channel (with large potholes) scoured into Precambrian Baraboo Quartzite; 2) huge quartzite breccia boulders and the presence of a quartzite breccia zone (26 km east of the classic Rock Springs outcrops); and 3) large "clay" cobbles embedded in Baraboo Quartzite.

The channel strikes N77^oE and is approximately 111m long with an undulating surface exposed along most of its length. An entire side of the channel scour is visible for 22 m and is 8.5 m high; the bottom of the channel side is at an elevation of about 295 m (967 feet). The channel side exhibits percussion marks, flutes, and a 2 m pothole at the top. The June 2000 blasting that destroyed this channel exposure revealed another channel 3 to 4 meters to the south that contains a pothole at least 3 m in diameter. All channel exposures are buried under the Cambrian Parfreys Glen Formation.

The breccia zone is located approximately 15 m east of where the channel exposure abruptly ends. It is a poor exposure and appears to be about 8 m wide. Within the breccia zone are pockets of dickite containing quartz crystals, some up to 13 cm long. Several well-rounded boulders and cobbles of breccia were observed in the quarry talus, but one impressive breccia boulder almost 3 m in diameter was found in place near the east end of the channel exposure. It is clearly part of the Parfreys Glen Formation.

Below the channel scour exposed by the June blasting is a zone of Baraboo Quartzite that contains about three dozen well-rounded cobbles and small boulders of apparently phyllitic (pyrophyllite?) composition. A thin layer of phyllitic material underlies and intersects the channel scour.