Devil's Lake State Park’s bluffs are part of the Baraboo Range, which scientists believe were formed 1.6 billion years ago, making them one of the most ancient rock outcrops in North America. The Baraboo Range includes the North Range and South Range of hills, which surround a canoe-shaped depression called the Baraboo Valley. The city of Baraboo is near the center of the valley. The north and south ranges meet in the east (just west of Interstate Highway 90-94) and west (at Rock Springs, Wisconsin).
From Sand to Quartzite Rock
Devil's Doorway was created by water freezing and thawing in cracks in the rock.
DNR Photo
These ancient hills are formed of quartzite rock, which consists of grains of sand tightly cemented together. According to geologists, the sand was deposited by rivers as they drained into shallow seas covering this area a billion years ago. As the sand accumulated, it first formed sandstone (a porous sedimentary rock) and then, under great heat and pressure, became quartzite (a non-porous metamorphic rock).
The Ranges Rise
Some time after the seas withdrew, the quartzite was buckled upwards in such a way as to form the North Range and South Range, with a depression between the ranges. The depression was filled with rocks softer than quartzite.
The area was then dry ground for a very long time. During this period, the Baraboo Valley was formed as the soft deposits in the depression eroded away.
Parfrey's Glen is one such gorge. The glen was eroded by water action because the rock formation there is not quartzite, but a Cambrian sandstone with quartzite rocks embedded in the sandstone.
Underwater Again
New seas re-invaded this area and their sediments accumulated on the land outside the Baraboo Hills, on the sides of the quartzite ranges, in the gorges, and eventually on top of the Baraboo Hills. The gorges were thus filled and the bluffs completely buried under sandy and limey deposits.
After the retreat of these seas, an ancient river or rivers removed most of the sediments from the Baraboo Hills and the surrounding area, thus exposing the quartzite bluffs again, and reopened the Lower Narrows Gap and the Devil's Lake Gap (these gaps may have been partially cut when the Baraboo Valley and the gorges were being formed).
Exposed outcrops of the quartzite that weren't covered by the glacial ice pack were subject to freezing and thawing conditions. Water seeping into cracks in the quartzite expanded as it froze and eventually broke pieces of the bluff away.
The thawing and freezing cycles also formed the piles of broken rock called talus on the slopes of the bluffs.
The Ice Age
The final chapter in this fascinating story took place about 15,000 years ago, when a sheet of ice (the Wisconsin Glacier) crunched into this area. The glacier covered the eastern half of the Baraboo Hills with ice but not the western half. We know this because its outermost boundary is marked by a ridge called a terminal moraine. This ridge consists of rocks and gravel dropped by the glacier as it stood and melted along this boundary.
The Wisconsin Glacier rerouted the ancient river(s) elsewhere and deposited dams of rocks and earth at the two open ends of the Devil's Lake Gap. These damps are part of the terminal moraine. Devil's Lake therefore is between two glacial "plugs" in an abandoned valley of an ancient river.
If there had not been a Wisconsin Glacier, presumably the ancient river(s) would still be flowing through the Lower Narrows and Devil's Lake gaps, and there would not be a Devil's Lake.
