Excavation reveals unique geologic formation
Small boulder of tillite with caliche (calcite) fluorescing white, with yellow-fluorescing dolostone under ultraviolet light. Color versions of these photos available at pdccourier.com.
Small boulder of 2 billion year old metamorphic gneiss carried in by the glacial ice from the Canadian Shield of northern Minnesota/southern Canada.
Glacial tillite containing cobbles of local Ordovician dolostone and dark pebbles of Precambrian volcanic rocks from the Canadian Shield.
By Phil Burgess
This past October, workers digging a hole for a basement foundation on a ridgetop near Bridgeport were amazed when they struck a tough layer of what seemed to be concrete about 6 feet beneath the ground surface. This material was so hard, it had to be jack hammered into pieces in order for it to be removed. The landowner called me and asked if I’d come out and have a look at what they’d dug up and piled next to the hole.
I was surprised at what I saw when I arrived there as well. So I did some research and also contacted a geologist professor/friend. We determined the “concrete” was actually glacial till (tillite) that had been cemented into a jumbled mass by tough caliche (hardpan) deposited by groundwater. The caliche and some of the dolostone fluoresce bright white and yellow under UV light in the dark.
Geologically, the tillite consists of mostly angular blocks of local Lower and Upper Ordovician-age (about 490-450 million years old) dolostone and limestone, with rounded pebbles and cobbles of Precambrian-age volcanic and metamorphic rocks (basalt, granite, etc.) derived from the Canadian Shield lying hundreds of miles to the north. The only fossils I noted were shell fragments, a stromatolite and a trilobite “tail.”
The deposit is what geologists term “pre-Wisconsinan age” and was formed by a narrow lobe of glacial ice that advanced eastward across this portion of the Mississippi Valley from Iowa between 800,000 and 2 million years ago. It would have taken unimaginable force to fracture and crush the dolostone bedrock in such a manner. The deposit is not related to meteorite impact, volcanic action or local karst.
Although Crawford County lies within the Driftless Area, the latter term only applies to the most recent glacial stage—the Wisconsinan—which bypassed all of Crawford County and immediate surrounding regions. Some of the earlier glaciations actually did impact parts of the local area, and large boulders of ancient Precambrian rocks start showing up at LaRiviere Park.
The temporary excavation is now filled in, but the contractor hauled two dump truck loads to the Driftless Area Wetland Centre in Marquette, Iowa, for the public to view. Thanks to the landowner, contractor and Driftless Area Wetlands Centre, this was possible.
