As the sun sets in the west and lights up Grandad Bluff in a stunning display, do you ever wonder about the story behind those layers of rock? When were they formed? Where? What makes up those layers? For many of us, these questions rarely surface, but to a geologist looking at Grandad it is like reading a journal that Earth has kept through millions of years.
Though I am certainly not a geologist, the story unfolds nicely in books such as Wisconsin's Foundations by Gwen M. Schultz and Roadside Geology of Wisconsin by Robert H. Dott, Jr. and John W Attig. Millions of years ago, Wisconsin was south of the equator just off the coast of an ancient slab of Earth's crust and mantle called Laurentia as shown by the black arrow on the diagram below.
This slab, or craton, was one of several that rode the tectonic plates through the oceans like bumper cars, smashing into each other to form super continents like Gondwana and Pangea, then breaking apart into individual continents like we have today. The remains of Laurentia now forms the core of North America, but over 500 hundred million years ago it was adrift south of the equator inching its way towards Gondwana. Laurentia also had mountains called the Penokees, or, as some call them, "Wisconsin's Rockies". These mountains were indeed impressive, towering high above sea level. Over millions of years, however, even these seemingly indestructible mountains were whittled away by erosion and reduced to their present remains in the northern parts of Minnesota and Wisconsin extending into Ontario. These remains are called orogens which means a place where mountains previously existed but were eroded down to their roots. Given enough time, the saying goes, "water wins", even against the most solid of rock.
As these mountains were slowly eroded various types of sediment were carried into the oceans off the coast of Laurentia where Grandad Bluff and the rest of Wisconsin were built layer upon layer like a cake being made by an ancient chef. Sea level varied considerably over these millions of years causing the future Wisconsin to be at different depths in the ocean off the coast of Laurentia. This caused different types of sediment to be laid down at different times, with fine materials settling out in the deeper, calmer waters when Wisconsin was further from the coast and the bigger grains of rock and sand being deposited in the more turbulent waters when it was closer to the shore. Grandad Bluff is the result of these millions of years of building as shown in the following diagram:
In the February 2024 issue of Scientific American there is a fascinating article, "Sand Mafias", about the ever growing need and scarcity of sand to make concrete. Sand mining is the largest extraction industry in the world today. But the type of sand that is used for concrete requires more angular grains for higher quality. The sand in the Driftless Area, however, including in the layers of Grandad, is more rounded, having had the sharp edges rounded off by the constant churning in water. This feature, along with others, makes it excellent for fracking, hence the frack sand mines that exist around here. The article even says that sand carries features that are almost like fingerprints and that optical systems have been developed that can pinpoint the origin of most sands.
Now, let's take a trip up Bliss Road. The first rock outcropping that is seen on the right across from the first parking area is likely from the Wonewoc Sandstone Formation:
A small piece chipped off one layer then ground up looks like this:
Further magnified like this:
Note that the edges of the grains of sand, which are small pieces of quartz or silicon dioxide, look kind of smooth. Interspersed are darker pellets which is probably a mineral called glauconite. Maybe this sand has its own "fingerprint"!
On to the next outcropping which is located on the right side of the road as it takes a sharp turn to the right:
Obviously, this photo has several different layers indicating different conditions, especially the depth of the water, in which they were formed. I believe the center layer is part of the Tunnel City formation with a prominent laminated green layer that is sometimes called "greenstone" or glauconite as in the previous photo. The layer just above is a very soft sandstone that might be part of or similar to the Jordan Sandstone layer and looks like this under magnification:
Further up the road, just before the Alpine Inn there is a rock outcropping on the right that includes dolostone, a very hard stone that was mined on Grandad Bluff before it was put to a halt by the efforts of Ellen Hixon and others over 100 years ago:
Dolostone is a very hard sedimentary rock that formed in the past under specific conditions that do not exist today and therefore is not currently forming anywhere in the world. It has magnesium incorporated into its calcium carbonate chemical structure turning the softer limestone into the hard dolostone. It is very resistant to erosion and caps the tops of the bluffs in the La Crosse area. Indeed, if it weren't for the dolostone protecting the underlying sandstone, we would probably not have many bluffs at all!
Which brings us to one focus of the efforts of Friends of the Blufflands which is the restoration of remnant prairies. Though old themselves, the prairies are babies compared to the layers of sedimentary rock on which they grow. The prairies in Hixon and the surrounding bluffs are sometimes called Dolomite and/or Sandstone Colluvium Bluff Prairies as described in this USDA publication, Ecological Site Description, Major Land Resource Area 105, Northern Mississippi Valley Loess Hills: https://efotg.sc.egov.usda.gov/references/public/WI/105XY001_DolomiteColluviumBluffPrairie.pdf. Here is a diagram from this publication of this type of prairie:
As described in the article, the prairies in Hixon Forest and other nearby bluffs grow on dry, steep, south to west facing slopes. The bluffs and valleys in the Driftless Area are, of course, products of erosion themselves. Over the years pieces of dolostone break off and deposit themselves in small to large pieces on the slopes below. Those that collect on the prairies themselves can be seen best after a burn like the one done recently on Lookout Prairie:
These prairies have a unique plant community that thrives on the layers that make up the bluffs.
So, as you gaze up at Grandad Bluff, appreciate the events that occurred long ago to give us this magnificent icon right here in La Crosse!