Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
The oldest rocks in northeast Wisconsin are metavolcanic and granitic rocks related to the Penokean Orogeny about 1800-1900 million years ago. Following the orogeny, there was a quiet interval of erosion and weathering during which thick accumulations of quartz sand formed. This interval is called the Baraboo Interval. In addition to the Baraboo Quartzites, other quartz sandstones were deposited, including the Waterloo, Rib Mountain, Necedah, Barron, and McCaslin Quartzites. These quartzites are probably related to the Sioux Quartzite of Minnesota and South Dakota. There was also rhyolite volcanism in southern Wisconsin.
The quartz sandstones were deformed and metamorphosed around 1600 million years ago. This event was probably related to an orogeny in the western and central U.S. called the Mazatzal Orogeny. In this part of northeast Wisconsin, the rocks were folded into a large syncline. Later invasion by the Wolf River Batholith has obliterated much of the syncline. The northern side is represented by the long quartzite ridge of McCaslin Mountain, and the eastern end by the quartzite at Thunder Mountain. On the southern side, a thin metaconglomerate layer, the Baldwin Conglomerate (actually metaconglomerate) is believed to be a remnant of the quartzite.
|A large rock knob about 200 meters north of County W is a good exposure of Hagar "rhyolite."
Below: Views looking south from the knob.
|Left: Close-up of the "rhyolite." Despite the name and texture, there is no evidence for a volcanic origin for this rock and most geologists now consider it a near-surface intrusive rock, part of the extreme north end of the Wolf River Batholith.|
|Right and below: Close-ups of the rock.|
|Looking down from the knob to the southwest|
|Outcrops of "rhyolite" are common below the knob|
|A faint trail shows that hunters and field trips regularly visit this spot.|
|Looking southwest toward the base of the hill.|
|Left and below: A large, classic erratic sits on the first rock bench above County W.|
|Left and below: close-ups of the Baldwin Metaconglomerate. Pebbles of granite and Waupee Volcanics occur in the metaconglomerate.|
|Rocky outcrops about 200 meters south of County W are Waupee Volcanics.|
|General appearance of the Waupee Volcanics|
|Left and below: In this area the Waupee Volcanics are strongly deformed and foliated. The deformation, which extends for several miles more or less parallel to County W, has been called the Mountain Shear Zone.|
Created 04 May 2005, Last Update 09 January 2020