An elongated ring of hills and ridges rise in south-central Wisconsin. Known as the Baraboos, they represent outcrops of ancient Precambrian quartzite, 1.5 billion years old. Folded downward by tectonic forces and buried by younger Paleozoic sediments, erosion has since exposed the edges of this geologic syncline; due to the tilt of this quartzite basin, its southern rim is far more prominent than its northern edge. The primary axis of the range, 30 miles in length, is oriented west to east; the shorter north-south axis is about 10 miles in length at the center of the Baraboos.
Between the north and south ridges is the Baraboo River Valley; flowing west to east (where it joins the Wisconsin River), the Baraboo and its tributaries have sculpted a landscape of hills and buttes from the Paleozoic sediments that filled the inner gap of the syncline. The river also passes through gorges (the Upper and Lower Narrows) in the northern Baraboo Ridge; geologists believe that the Wisconsin River initially sculpted the Lower (eastern) Narrows (see below).
Adding to the geologic complexity of the Baraboos is the presence of a terminal moraine, deposited by a lobe of the Wisconsin Glacier late in the Pleistocene. Bisecting the ranges from north to south, the glacier and its moraine blocked the course of the Wisconsin River, diverting its flow east of the Baraboos and leaving Devil's Lake in the river's abandoned gorge through the southern Baraboo ridge.
