FERRYVILLE - Geologist Dr. Eric Carson took a captivated audience of almost 50 at the Ferryville Chautauqua presentation on July 10 on a geologic trip through time.
Carson’s specialty is research about the processes and sedimentary archives of streams near former glacial margins. In his 11 years working with the Wisconsin Geological and Natural History Survey, Carson has particularly focused on the Upper Mississippi and the lower Wisconsin River basins.
“Geologists are fascinated by the Driftless Region because it was never covered by glaciers and provides us with a window into time,” Carson explained. “The size of this unglaciated area is literally unique on the planet.”
The unglaciated Driftless Region, Carson told the group actually only encompasses the area in Wisconsin and a little corner of Jo Daviess County in Illinois. Although the areas across the river look very similar to those in Wisconsin, those areas in Iowa and Minnesota were actually glaciated somewhere between a half and two-and-one-half million years ago.
Like the Wisconsin areas, they were not glaciated by the two most recent glacial periods. Those two are referred to as the ‘Illinois glacial period,’ which took place about 100,000 years ago, and the ‘Wisconsin glacial period,’ which took place about 30,000 years ago.
Carson fielded a question from the audience about why the two most recent glaciers had missed the Driftless Region. He explained that the Wisconsin glacier had flowed from the northwest to the southeast, and missed the area on its eastern side. The Illinois glacier, which stretched as far south as St. Louis, had flowed from northeast to southwest along the Lake Superior and Lake Michigan basins.
“The Lake Superior basin was deeper, and was oriented perpendicular to the glacier, so the glacier was impeded by that as well as by old igneous rock in northern Wisconsin,” Carson explained.
Drifttless research
Recently, Carson’s work has taken him to such local areas as Stoddard, Coon Creek, Rush Creek and Copper Creek in rural Ferryville. Private landowners and the Wisconsin Department of Natural Resources have allowed Carson and his team to drill core samples in those areas. Along with the core samples, Carson’s research also employs Light Detection and Ranging (LIDAR), which is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth.
“Use of these two technologies allows geologists like me to see the dominant geologic processes that formed the river’s basin,” Carson explained. “LIDAR gives us maps that are accurate within one centimeter.”
In particular, Carson is interested in several odd characteristics of the Driftless Region. First, the floodplains of the Mississippi and Wisconsin rivers are so wide and flat; second, the bluffs along the Mississippi are so straight up and down, and the river valley itself is filled with 200 feet of unconsolidated material above the bedrock floor; and third, there are several terraces which show, from sediment, how high the river used to be. Those two terraces are the Bagley Terrace (Bagley, Stoddard) and the Savanna Terrace (up into the valleys of tributaries).
Rivers and glaciers
“Rivers are conveyors of water and sediment, and this will cause river levels to bounce up and down as sediment settles out and then gets washed out at different times,” Carson explained. “These cycles will cause river levels to rise and fall.”
Carson said that river levels build up during glacial periods because glaciers deposit large amounts of sand and gravel sediment. These materials are deposited down on deep incisions into the bedrock caused by earlier glacial periods. Later as the glaciers melt and recede, the sediment is moved downriver by the glacial melt, the river levels fall, and the sediment dries and is blown onto nearby uplands.
Carson said that on the highlands above the east side of the Mississippi River, in Vernon, Richland and northern Crawford County, the deposits of blown sediment or ‘loess’ can be as deep as 40 feet. Under those layers are found 20-25 feet of the ‘Rountree Formation’ clay. He said the layer of loess extends as far east as Madison, decreasing in depth the further east you go.
“The bigger river basins erode first, and then the smaller tributaries race down the landscape to catch up with them,” Carson said. “One thing that has particularly fascinated geologists is that all of the tributaries on the east side of the river between LaCrosse and Prairie du Chien are much smaller, shorter, and younger than those on the west side of the river or on the lower Wisconsin River.”
Carson explained how a rock formation along the Wisconsin River, between Prairie du Chien and Muscoda, gives clues about the past configuration and direction of flow of the Wisconsin River.
Carson explained that this formation is evidence that the Wisconsin River used to flow east and north, originating in what is now the Turkey River in Iowa, which flows from northwest to southeast from Cresco, Iowa. The river exited to the ocean through the St. Lawrence Seaway. Carson said geologists refer to this ancient body of water as the ‘Wyalusing River.’
“The Mississippi River actually did not flow south between LaCrosse and Prairie du Chien in the same time period,” Carson said. “Instead, the river flowed east from Minnesota, through northern Wisconsin and drained into the St. Lawrence Seaway.”
Carson said that the Wisconsin glacier came down into northern Wisconsin from the northeast, it blocked the flow of the Wyalusing River and created damned inland lakes, including Glacial Lake Wisconsin. The lake built up and accumulated right on the edge of the Driftless Region, near the foot of the Baraboo Bluffs.
Finally, Glacial Lake Wisconsin built up the backed up water from the then east-flowing Wisconsin River to the point that it spilled over, and created the Black River, which flowed to the Mississippi River near LaCrosse. The velocity and volume of this large quantity of water caused the Mississippi River to turn south, and created its current, south-flowing channel, between LaCrosse and Prairie du Chien.
When the Wisconsin Glacier began to retreat and melt, the 4,400-square-kilometer Glacial Lake Wisconsin drained catastrophically to the south, creating the Wisconsin Dells in just one week. Carson said that the flood that resulted is estimated to have been 20 times greater than the flood measured at Muscoda in 1938. The draining of the glacial lake caused the Wisconsin River to reverse flow, and flow west to join with the Mississippi River at Wyalusing.
Carson says his current research is looking at the sediment that was left on different terraces as a clue about the history of the river and the formation of its tributaries.
“As glacial outwash was deposited in the river valleys, it blocked the mouths of tributaries and caused small lakes to form,” Carson explained. “The Bagley terraces, such as one located at Stoddard, are lower down right at the level of the river, versus the Savanna terraces, which are located up higher into the valleys of the tributaries.”
Carson said that the Bagley terraces are mostly composed of sand and gravel. The Savanna terraces in the valleys of the tributaries contain fine-grained lake sediment with plant material mixed in. Through radiocarbon dating, Carson is able to clearly identify the age of the layers in his core samples taken in the Savanna terraces, giving him additional clues about the geologic age of those layers. Carson has taken Savanna terraces cores in both the Coon Creek and Rush Creek watersheds.
