Karst weathering creates unique formations
An example of karst-weathered rock from the Driftless Region. Several fossils are present throughout the specimen, preserving evidence of several species present in the area hundreds of millions of years ago. (Steve Van Kooten/Courier Press)
By Phil Burgess
During my younger days, I spent a lot of free time combing the timbered bluffs of Crawford County in quest of its ancient fossils.
During the time period of Earth’s geological history known as the Lower Ordovician, the entire Tri-State Region was submerged beneath a widespread shallow tropical sea known as the Iapetus Ocean. Thick layers of limy sediment were slowly deposited in this shallow sea. Over time, this sediment hardened into dolostone, which geologists named the Oneota Formation. This rock formation has been dated at roughly 485 million years old, so it is an ancient rock formation.
Bear in mind that this was long before dinosaurs existed, which was between 252 and 66 million years ago.
The Oneota Formation contains a lot of chert, which is a very hard, siliceous rock that occurs as nodules or layers in the massive dolostone. It has been exposed to deep weathering processes for hundreds of millions of years, both from surface weather by ice and rain and from subsurface karst solution by groundwater.
The photo is of a karst-weathered Oneota Formation that I hauled out of the wooded hills of central Crawford County about 15 years ago. The friendly landowner even treated me to dinner after I showed it to him and told him what it was. I explained that it was composed of dolostone and chert and that had likely been slowly eroding away on his hillside since before the last episode of widespread glaciation — which had bypassed the region.
The softer carbonate dolostone dissolved, leaving behind the more resistant chert. The end result was this peculiar shaped natural “objet d’art.” The spongy-looking surface contains a number of fossils, including gastropods (snails) and chambered cephalopods. It had broken off of a large stromatolite reef concealed inside the bluff. This type of karst weathering is unique to the Oneota Formation of our Driftless Region.
