A Journey of Millions of Years
Updated: Apr 29, 2022
On Saturday, June 12, several intrepid time traveling adventurers left their homes and went on a journey of millions of years. Many started their journey through time at 12,000 years ago then were instantly transported back in time to about 550 million years ago. They then had a more leisurely climb advancing through about 50 million years, then bang, back up to 12,000 years ago. Along the way they passed Eau Claire, Wonewoc, Lone Rock, Jordan, Oneota, then reached their destination at Loess. Sounds crazy, right?
Well, the journey started from the sandy terrace of La Crosse, laid down about 12,000 years ago after the last glacier, then up the bluff climbing through the geologic layers laid down millions of years ago in the Cambrian and Ordivician periods, then back on soils called loess that blew in and covered the tops of our bluffs, including Grandads Bluff, again at about 12,000 years ago. The trip was set up by the local chapter of The Prairie Enthusiasts with some members of Friends of the Blufflands also attending. The purpose of the trip was to learn about the soils and geology of bluff prairies. We chose Zoerb Prairie as the one to study. Peter Hartman, a retired soils scientist from the USDA, led the trip.
The loess forms the top layer on our bluffs. Think of it like frosting on a cake. It is up to 3-4 feet deep on the very top and was blown in by winds after the last glacial period ended about 12,000 years ago. It is considered a very fertile soil. We walked a short distance from near the weather station, crossing the road FA where we took our first core soil sample. There we found a very thin layer of top soil above the subsoil, an indication of past erosion. We then proceeded to Zoerb Prairie itself. The very top of the prairie, along a spur trail that goes a short way onto the prairie, is a hard rock called dolostone. This rock is very resistant to erosion and forms the caps on all of the bluffs surrounding La Crosse. Dolostone is like limestone, but it formed in magnesium carbonate rich seas and has magnesium incorporated into its chemical structure. Without this protective rock, most of the surrounding bluffs would have eroded away by now. Then onto the prairie itself and, like most undisturbed prairies, the soil was in much better shape with the rich black top soil being at least 10 inches deep. As we proceeded down the prairie we thought we might find evidence of the Jordan sandstone layer, but this was not at the surface being covered by a mixture of dolostone rocks and loess from the top called a loamy-skeletal colluvium. This is a mixture of loess, broken down rocks and intact rocks from above. The next surprise was to see a geologic layer called the St Lawrence formation which forms a cliff edge at the base of the prairie. I have climbed around on this cliff edge many times while working on Zoerb and have wondered if this was the St Lawrence. It was nice to have this confirmed, and Peter seemed excited as well saying it was the first time he had seen it so well defined. Next was the Lone Rock formation which is a sandstone layer known for its green hues from glauconite. Last was the Wonewoc sandstone layer seen as light brown sandy deposits at the base of the prairie.
We then climbed back to the top of the prairie again admiring the many beautiful prairie plants growing there and having a better understanding of the different layers and soils on which they thrive. The trip was very enjoyable and didn’t seem to last much time at all- way less than millions of years!