Devil’s Lake Geologic History:

Location:

Devil's Lake is in Baraboo Wisconsin.  The lake is located in a portion of what is called the Baraboo hills.  From an aerial view these hills of south-central Wisconsin look like an oval ring, 30 miles long and 10 miles wide.

Composition/Structure:                    

Devil's Lake is flanked on its east and west sides by the ancient Baraboo hills.  The Baraboo hills are composed of metamorphic quartzite (specifically -Baraboo Quartzite), a rock of great strength.  Over the past 350 million years, the forces of erosion (not the least of which were glaciers) have worked their magic; the once covered hills have been exposed as the overlying material has been stripped away.  A cross-section of the range shows each side of the oval represents the "arm" of a syncline (a fold in rocks in which the strata dip inward from both sides).  As if this area's geologic uniqueness were not enough, the section of the hills that include Devil's Lake was directly impacted by the last Ice Age.  The terminal moraine of the last glaciation left material at each end of an ancient river valley creating Devil’s Lake.

Formation of the bluffs (the quick version):

1.5 billion years ago the landmass that makes up Wisconsin was not land locked like it is today.  It was on the edge of the ocean; this ocean front view helped create a sand beach.  That sand beach was frozen in time by being exposed to intense heat and pressure (you can still see the ripple marks in some of the rocks that were created by wave action in the sand 1.5 billion years ago!).  That intense heat and pressure helped turn that once sand beach into metamorphic quartzite.  This metamorphic quartzite was compressed by colliding continents and formed a syncline (see the definition above, similar to a folded piece of paper that points downward, a picture is provided below).  Over time, material covered and then ultimately eroded to uncover the metamorphic quartzite once again.  The quartzite bluffs at Devil’s Lake rise above the water by 450-500 feet.  Prior to ice wedging and other erosive forces they were roughly twice as high.  The eroded bluffs have created large talus piles at the base of each bluff.


photo credit:  http://www.physicalgeography.net/fundamentals/images/syncline.gif

Formation of the lake (the quick version):

An ancient river carved its way between the portion of the bluffs that make up present day Devil's Lake.  Exactly which river flowed between the bluffs is still debated today.  This section of the river was trapped between the bluffs by glacial till that was deposited by the last glaciation (13,000 years ago).  More specifically, the terminal moraine of the glacier deposited material at each end of the bluffs trapping the water and creating present day Devil’s Lake.

 

Why are the rocks red? (the quick version):

To unravel the mystery of why the rocks have a reddish pink coloration you must go back to the beginning of this story, 1.5 billion years ago.  Shortly after the sand was pressure cooked to create the metamorphic quartzite, there were sticky mats of iron eating bacteria living on the rock.  This bacteria was not only responsible for the development of some of the early chemistry of life, but was also responsible for staining the quartzite red.


 photo credit:  http://www.geo.umn.edu/orgs/umgs/baraboo/115_1518.jpg