These hills were formed from deposited sediment in a shallow sea 1.7 billion years ago. This sediment was metamorphized into quartzite, deformed, then uplifted. These outcrops may have been islands when the region was covered by shallow seas in the Cambrian. The
Baraboo River divides the range in half, flowing through Upper Narrows Gorge near
Rock Springs and travels onto its confluence with the
Wisconsin River downstream from
Portage through the Lower Narrows. The Baraboos are composed of resistant
Precambrian quartzite (a
metamorphic rock) which has formed an erosional remnant or
Monadnock, resulting in
topographic prominence. The mountains may have formed as long ago as the late Precambrian. These formations were buried by
Paleozoic sedimentary strata and are still being uncovered by the erosion of the softer, overlying rocks.
Devil's Lake, the centerpiece of
Devil's Lake State Park, was formed from
terminal moraines blocking access to its outlet, creating what is today an
endorheic lake (i.e., a lake lacking a surface outlet to the world's oceans). In addition to Devil's Lake State Park, significant sections of exposed quartzite can be found at
Ableman's Gorge State Natural Area and Lower Narrows State Natural Area. Ableman's Gorge is a natural gorge featuring unconformable contacts between the quartzite and younger Cambrian
sandstone, exposed through a combination of erosion and modern quarrying. Bluffs at Lower Narrows carved by the Baraboo River expose contacts between the quartzite and younger volcanic
rhyolite. ==References==