David Horst column: Formations tell of Wisconsin volcanoes and tropical seas
Known today as Mosquito Hill and Cactus Rock, respectively, Lawrence University geology Prof. Marcia Bjornerud sees them for what they were ages ago and marvels at them the same way she does her research sites in Canada and Norway. Bjornerud, author of the recently released book “Reading the Rocks: The Autobiography of the Earth” (Westview Press, 2005), Saturday will lead a tour of the two local geological wonders for the Fox Valley Group of the Sierra Club. Bjornerud has twin passions for rocks and words. Her book flows like a historical novel and would hold your interest even if all you remember of geology is that rocks come in sedimentary, metamorphic and that other one. Mosquito Hill, just east of New London on County S, is sedimentary — a preserved bump of limestone that held a shallow tropical sea before continental drift moved Wisconsin to a higher latitude. Bjornerud said its granite cap spared it from the glaciers that covered the hill.
Much smaller but more fascinating an anomaly is Cactus Rock, named for the honest-to-goodness native Wisconsin cacti that grow there. (More on that in my next column).
I accompanied Bjornerud on a class trip there last week. The site, known as Poppy’s Rock to the locals for the family that once owned it, is a large outcropping of the pinkish volcanic rock rhyolite lying just south of New London. Lawrence owns it and uses it as an outdoor classroom.
Although it has not been scientifically dated, Bjornerud estimates Cactus Rock at 1.7 billion years old. She postulates that it is related to more extensive rhyolite formations farther west in Berlin and may have fed massive volcanoes there.
After allowing her students to scramble over the rock making their own observations, Bjornerud explained that this close-grained, granite-like, igneous rock is a visible bit of Precambrian history.
“It’s a peculiar little exposure of this rock in a sea of much younger rock,” she said.
Created like Mount St. Helens by violent eruptions, it was buried and then exposed by more than a billion years of erosion.
“You can walk on the Precambrian topographic surface,” she said with undisguised awe. “That, to me, is eerie.”
That landscape was rounded by the glaciers, the scrapes known as striations and chatter marks still visible. Cactus Rock also bears the graffiti marks of its more recent history as a high school party spot.
“It’s been an attraction for many, many years,” said Neil Freeman, who lives nearby. It was a traditional gathering place for parties for graduating seniors.
He remembers the area being logged off in the late 1960s, after the Poppy family sold it to American Plywood Co. of New London. The company later donated it to the Nature Conservancy, which deeded it to Lawrence.
Mosquito Hill also has an alternate modern history from before it became the county-owned nature center it is today. Freeman watched ski jumpers launch off of the north slope and motorcycles race up the west face of the hill in the late 1940s.
On Bjornerud’s time scale, Mosquito Hill’s days are numbered. Erosion is still wearing it away.
“It’s on its way out,” she said. “It’s a remnant island.”
Luckily for those of us who enjoy hiking there, the demise of Mosquito Hill will be in a far distant chapter of Earth’s autobiography.