Tom's Blog

Wednesday, November 11, 2015

Driftless area geology at Pleasant Valley Conservancy

One of the reasons Pleasant Valley Conservancy (PVC) is so scenic is because it is in the "heart" of southern Wisconsin's famous Driftless Area. The rugged hills exist in our area because they are capped with an erosion-resistant rock type called the Oneota Dolomite.

Underlying the Oneota is a less erosion resistant formation called the Jordan Sandstone. Both the Oneota and the Jordan have outcroppings at Pleasant Valley.

Back in 2002 we were fortunate to have geologist Bob Dott visit PVC and create for us a geological cross section. Bob and I covered the whole Conservancy and he located the various rock layers. 

Among other things, Prof. Dott discovered a contact point between the Oneota and the Jordan along the side of the Ridge Trail that descends from the Far Overlook. The attached map shows the approximate location of this contact. (It is permissible here to take a short exit from the trail in order to see this geological location. Take care, because the slope is very steep here!)

ArcGIS map of the west end of PVC, showing the various trails.

The easiest place to see the Jordan Sandstone is the large outcrop in the Quarry. (See photo) The reason the Quarry exists is that about 50 years ago the Jordan Sandstone removed here was use to create Cedar Hill Lane, just across County F. The trail from the Quarry to County F follows the old road used to bring the rock down.

The rock face consists of the Jordan Sandstone. See map above for location where this photo was taken.

Geology is important in controlling the ecology of PVC. Because the bedrock is so close to the surface, it plays a major role in soil formation. Because the Oneota dolomite is rich in calcium and magnesium carbonates, its overlying soil is calcium/magnesium-rich and alkaline, making it favorable for calcophilic plant species such as buckthorn. Soils formed on the Jordan, well seen at the east end of PVC (the East Basin, especially) however, are quite acid, with pH values around 5.0 to 5.5.


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