discovery
91° W
90° W Baraboo Hills
Mi
ss.
Riv e
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43° N
L. Wisconsin R iver
Military Ridge 0
LiDAR-derived image of the lower Wisconsin River valley and its confluence with the Mississippi River. The geomorphological clues that indicate drainage reorganization has occurred include the barbed tributaries of the lower Wisconsin River (pale blue arrows); the curved inner valley wall at the confluence of the Wisconsin and Mississippi Rivers (solid orange line), which is similar to the inside of a bend on a single river (example identified by dashed orange lines); and the narrow reach of the Mississippi River immediately downstream from its confluence with the Wisconsin (yellow bracketing arrow). The white boxes indicate the three remaining segments of the terrace where Carson and his team took 63 core samples.
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the Mississippi River also narrows. These geologic clues suggest that the modern river system does not reflect the system that once flowed across this landscape. The westward-flowing Lower Wisconsin River may not be exactly what it appears at first glance, and the bedrock terrace contains the key to discerning what river systems looked like here millions of years ago. While tens to hundreds of feet of glacial till obscure most of the river’s older landscape, the Driftless Area has remained untouched (yet very much surrounded) by glaciers over the past 2.5 million years. The bedrock terrace, preserved within this time capsule of sorts, allows me to ask a fairly direct question of the earth: Was the original valley carved by a river flowing toward the west or toward the east? It’s a question as intriguing as it is unique, since in my experience it is rare that an important scientific question offers only two possible outcomes.
A
s a geologist, I specialize in studying the processes and histories of rivers; more specifically, I study rivers that flow near the former margins of ancient glaciers. The idea that glacial activity may have caused the ancient Wisconsin River to flow to the east grew from many discussions I had with my former doctoral advisor, James C. Knox, before he passed away in 2012. Jim was the EvjueBascom professor emeritus of Geography at the University of Wisconsin–Madison and a pioneer in stream and soil research. Over the years, he and I tossed around hypotheses about the Wisconsin River that couldn’t be tested because of insufficient topographic data. In the early spring of 2013, however, improved topographic models, based on a new generation of laser-derived data called LiDAR, became available. Generated by measuring the two-way
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WISCONSIN PEOPLE & IDEAS