If a dinosaur fell into the ancient Mississippi River near what is now La Crosse, Wis., it might have been carried north and west by the Mississippi and dumped into the Arctic Ocean, according to a new theory about the river’s origin.
The theory, developed by Howard Hobbs, a senior scientist at the University’s Minnesota Geological Survey, contends that the Mississippi north of La Crosse once flowed north into the Minnesota River and then west into the Red River.
Hobbs presented his theory at a recent conference of the Geological Society of America in Madison, Wis.
“Many people don’t realize that even today the Red River flows north,” Hobbs said.
According to Hobbs’ theory, glaciers advanced south, and they blocked the northward flow of the Mississippi, causing much of Minnesota and Wisconsin to be covered by a shallow lake. The lake stretched from the front of the glacier to the Paleozoic Plateau, a broad dome of bedrock that peaks near La Crosse.
By the end of the ice ages, about one million years ago, the water in the lake found an exit south through a low spot in the plateau, carving out a new river channel. Deposits left by the retreating glaciers, notably the Big Stone Moraine in southwestern Minnesota, prevented the Mississippi from reclaiming its old northward course.
Hobbs developed his theory to resolve a paradox first noticed by geologists nearly 100 years ago.
As the Mississippi flows south from the Twin Cities, the bedrock bluffs on the side of the river get higher and higher. This geological oddity, Hobbs said, would suggest the river should have flowed north.
If the Mississippi originally flowed south on the surface of the rock, Hobbs said, it would have flowed uphill for about 100 miles.
But other University geologists are not so sure that a flow reversal is required to explain the river’s present course.
“I think that’s pretty much explained by classical models that have been around for a hundred years,” said Roger Hooke, a geology professor.
The conventional view, based on the work of 19th-century Minnesota geologist Newton Horace Winchell, is that there was originally a layer of softer rock on top of the current bedrock that sloped south and eroded away during the ice ages by the glacial run-off.
“On top of the existing rocks which form the bluffs, there were other rocks which were not cliff formers — they were softer rocks,” said Herbert Wright, a regents professor emeritus of geology. “This is all rather speculative because there is no positive evidence for these (softer rocks).”
Between one and two million years ago, glaciers repeatedly advanced into Minnesota and then retreated during warm spells. Run-off from retreating glaciers increased the flow of the Mississippi and carved out deep gorges, Wright said.
Hobbs discounts the idea that there was once an additional layer of rock on top of the current surface.
“The problem with that is that if you were to try to reconstruct that surface, it would’ve been hard, resistant rock that would have been difficult to remove by erosion,” he said.
For the past 15 years, Hobbs has been mapping Minnesota’s buried river valleys in an effort to reconstruct pre-glacial water flow.
“The bottoms of most buried bedrock valleys slope toward the north in southeastern Minnesota and in the Twin Cities area,” he said. “But near the Iowa border, several river valleys flow south.”
Hobbs’ theory concurs with other studies that suggest rivers currently flowing south once flowed north; one is a recent finding by geologists working for the North Dakota Geological Society. The society’s researchers concluded that the Missouri River used to join a northward flowing river in Manitoba — the ancient counterpart of today’s Red River.
During the ice ages, a glacier pushed the Missouri south, the North Dakota researchers say. The river flowed along the southern edge of the ice sheet and now flows along the edge of the deposits left by the glacier.