The rock here was sand on a sea floor 1.7 billion years ago, and it has been rising ever since.
The Baraboo Range runs about 25 miles across Columbia and Sauk counties in Wisconsin, between 5 and 10 miles wide. Geologically it is a syncline, a downfold of heavily eroded Precambrian metamorphic rock. The sediment was laid down in a shallow sea, metamorphosed into quartzite, deformed, and then uplifted, possibly as long ago as the late Precambrian. The US Geological Survey records a silica operation in Sauk County under the name Baraboo Operation, which is the quartzite in commercial terms.
Quartzite is the reason there are hills at all. It is a hard, resistant rock, and as the softer Paleozoic sedimentary layers that once buried these formations erode away, the quartzite is left standing as an erosional remnant, a monadnock with real topographic prominence. The uncovering is still going on.
There is a good chance these outcrops were once islands. When shallow seas covered the region again in the Cambrian, the Baraboo hills would have stood above the water.
The range also marks a boundary that matters across the whole state. Its eastern end was glaciated during the Wisconsinian glaciation and its western half was not, which makes the Baraboos the eastern edge of Wisconsin's Driftless Area, the country the ice sheets missed entirely.
Water cuts straight through it. The Baraboo River divides the range in half, running through the Upper Narrows gorge near Rock Springs and on through the Lower Narrows to meet the Wisconsin River below Portage. The Wisconsin River itself, having run north to south, turns east just above the range before swinging west again toward the Upper Mississippi.
The city of Baraboo sits in the middle of the valley, and the range was made a National Natural Landmark in 1980.
Quartzite (Baraboo Range)
Other places on the map within a couple of hours.
