By Amanda Eggert EBS Senior Editor
BIG SKY – When Karen Williams looks at the Big Sky landscape, she sees an incredible and dynamic mix of geological elements—rock glaciers, landslides, faulted and folded rocks, and even an igneous intrusion—in close proximity to one another.
Williams, who has a doctorate in earth science from Montana State University where she is now an adjunct professor, will give an overview of Big Sky’s geology in a free talk at 7:30 p.m. June 6, co-sponsored by Lone Mountain Ranch and the Madison-Gallatin chapter of the Montana Wilderness Association.
An igneous intrusion is roughly equated to a volcano that didn’t quite breach the surface of the rocks above it, and the one Williams is referring to is Lone Mountain, arguably the most striking landscape feature in the region.
Williams said one of the oldest landforms in the area is located near the Dudley Creek side of the Spanish Peaks fault. Those rocks, which are classified as Archean, are approximately 1.4 billion years old and pre-date life on Earth.
But on the other side of the same fault, near the Big Sky Resort Golf Course, lies the opposite end of the spectrum. “That’s limestone, and it’s much younger than the rocks that are just over the ridge in the Dudley Creek area,” Williams said.
During her talk, Williams will present zoomed-out images of the area without vegetation so attendees can get a better feel for the structure and composition of the rock below. She said that data, also known as bare earth imagery, allows people to get a better grasp of the geological forces at play. She likens the way rocks were faulted or folded to a canvas with more recent geological elements like landslides and rock glaciers akin to paint on the canvas.
“When you take off the vegetation and you look at the topography, you see landslides everywhere,” Williams said. Although “landslide” might bring to mind “mud slide,” not all landslides are instantaneous events with catastrophic consequences. “The more watery it is, the more likely it is to be a mud flow. … There’s different gradations depending on the material properties [and] how well it flows.”
Williams said that there are a lot of tightly-folded shales in the Big Sky area, which tend to be weaker rock layers prone to sliding out.
Williams’ fascination with Big Sky’s geology was encouraged by a geomorphology class from long-time MSU professor Bill Locke. After Locke retired, Williams took over teaching that class. Her June 6 lecture will be based off a field trip to Big Sky she used to take with her students.