Dolostone Speaks Softly...But it Rocks!

The bluffs in the Coulee Region, offering breathtaking views like those from Grandad and Lookout, owe their existence to dolostone. The climate would be significantly different without dolostone. The distinctive karst geology of the Driftless area, characterized by its underground caves and cold water trout streams, is largely attributed to dolostone. Numerous prominent buildings and the foundations of homes in La Crosse were built using dolostone. Much of the Mississippi River's banks were fortified with dolostone, and many of our roads were "macadamized" with it, meaning it was crushed and used as their base. But what exactly is dolostone, or is it dolomite? What does it "say" to a geologist? And what is the "Dolostone Problem" that has puzzled geologists for many years?

Many are aware that there are three fundamental types of rock: igneous, metamorphic, and sedimentary. Igneous rocks are “fire born” and come from the mantle deep beneath Earth's crust. When this molten rock or magma penetrates the crust and cools slowly beneath the surface, it forms granite, which sometimes becomes the countertops in our kitchens or the gravestones in our cemeteries. If the molten rock erupts onto the surface and cools rapidly, it is referred to as lava and forms basalt. Over 90% of all volcanic rock on Earth is basalt. The island of Hawaii is a large basalt mound created from a "hot spot" in the mantle, where the Mauna Kea volcano has intermittently erupted for millions of years and is taller than Mount Everest by nearly a mile when measured from its base on the ocean floor! However, even Hawaii is overshadowed by another lava and basalt mountain—Olympus Mons on Mars. This mountain is the tallest in the solar system at 70,000 feet, 2.5 times higher than Everest and over 13 miles high! Mars has a rigid crust without tectonic plates, allowing the lava to continuously flow through this relatively permanent defect in the crust, causing Olympus Mons to keep growing taller. Basalt can also be observed at night by looking at the Moon's dark areas, called "maria", which in the past were mistakenly thought to be watery "seas." It was later discovered that these dark areas are actually ancient lava lakes that erupted on the Moon's surface, forming basalt. Indeed, in its younger days, the Moon released lava onto its surface from a molten core when it was bombarded by asteroids during a period known as the Late Heavy Bombardment around 4 billion years ago, when the solar system and the Moon were still in their early stages. Remember the "Sea of Tranquility," where Apollo 11 landed on the Moon in 1969, and Neil Armstrong said, "That's one small step for man, one giant leap for mankind"? That wasn't a "sea," but an ancient lava lake! 

Metamorphic rocks are formed when igneous and sedimentary rocks are subjected to high temperature and pressure and recrystallize. One familiar example of a metamorphic rock is marble that comes from limestone that has been buried deep in the crust and exposed to high heat and pressure for millions of years. Another is quartzite which is formed by the metamorphosis of sandstone. A cool example of quartzite is at Devil’s Lake State Park near Baraboo, Wisconsin where ancient Precambrian quartzite pokes through more contemporary sedimentary rock to see the light of day. Together, igneous and metamorphic rocks make up 90-95% of Earth's crust. The rest is sedimentary rock. Dolostone is a sedimentary rock

An earlier blog post, https://www.friendsoftheblufflands.org/post/grandad-bluff-and-the-sands-of-time-1 , explored the geology of Grandad Bluff as a series of sedimentary rock layers. Sedimentary rock is most often created when preexisting rock is eroded and small pieces or "clasts" are carried by rivers into bodies of water such as the ocean settling out in layers. The size of the clast and the water's turbulence where it is deposited significantly influence the layer formed. Calm water is needed for clay deposition, resulting in shale. In more turbulent water, sand can settle and form sandstone (such as the Jordan Sandstone in Grandad Bluff), while pebbles and gravel can settle and form conglomerates in even more turbulent water. Stratigraphy is the study of such layered rock. North America has experienced six periods of submersion and sedimentation known as the "Great Sedimentary Sequences". Marcia Bjornerud, a geology professor at St Lawrence University in Appleton, recently authored Turning to Stone, discussing the first of these sequences, the Sauk Sequence. This engaging book is fun to read and reveals the dynamic geology of Earth with rocks "speaking" to the author to tell their story. It details that Grandad Bluff and the other bluffs in the Driftless Area formed during the Sauk Sequence over 500 million years ago when what is now Wisconsin was located south of the equator near Laurentia, the ancient core of North America. This sequence is named after the indigenous people living in Wisconsin where it is best represented. The late Richard Dott from the University of Wisconsin, mentioned in the previous blog post as a co-author of Roadside Geology of Wisconsin, is described by Bjornerud as "an eminent sedimentologist" who devoted much of his career to studying the Sauk Sequence. He is said to have considered it "nature's finest distillate- almost as remarkable as a pure single malt Scotch whiskey" and "the product of a never-to-be repeated moment in Earth's history" formed when sea levels were low for a prolonged period and when plants had not yet evolved to live on land. The Sauk sequence is beautifully represented by the bluffs in the Driftless Area, including Grandad Bluff. It includes all of the sedimentary rock layers laid down during the late Precambrian into the mid-Ordivician period, from about 650 to 460 million years ago. Dolostone, the final layer, tops this sequence like icing on a multilayered cake, protecting the more erodible sandstone layers beneath. When we hike from the lower Hixon parking lot starting on Hickory Trail to Savanna Trail then Vista and finally the spur trail to the Lookout Prairie overlook with its stunning view, we have traversed million years of geology and are at last standing on dolostone. Without this dolostone, these ridges and their views would have eroded away long ago.

While dolomite and dolostone are often used interchangably, dolomite is technically a mineral composed of calcium and magnesium carbonate, whereas dolostone is a rock consisting primarily of dolomite, but with other components mixed in like chert, quartz, and feldspar. Bjornerud finds dolostone "cryptic" and finds it "maddeningly uncommunicative...a rock that mumbles". She says, "I just couldn't understand what dolomite was saying". By this she means that, unlike other rocks that clearly tell their story, dolostone is hard to figure out. It "must occur clandestinely, in the subsurface". The formation of dolostone remains a topic of debate, known as "The Dolomite Problem". Dolostone is abundant in rock layers over 100 million years old, but not in more contemporary rock. Currently, there is no known location on Earth where dolostone is forming and it has been difficult to replicate its formation in laboratory conditions similar to Earth's surface. The mystery of how it formed in such vast quantities, making up 2% of Earth's crust, persists. The book describes various thoeries attempting to solve this mystery, including the "evaporative reflux model" whereby seawater becomes concentrated through evaporation, increasing its magnesium content, which somehow integrates into the chemical structure of limestone, CaCO3, transforming it into the mineral dolomite, CaMg(CO3)2, and eventually into the rock dolostone. Another thoery is the "biogenic" model suggesting that the dolostone forms through microbial activity, a process now believed to account for about 40% of Earth's minerals. Last, dolostone is thought to possibly form over long periods through "periodic dissolution" where the mineral dolomite crystallizes with imperfections and gradually grows as water repeatedly washes away these imperfections, a process that takes millions of years.

Dolomite gets its name from a French geologist Dieudonne Sylvain Guy Tancrede de Gratet de Dolomieu. Wow, what a mouthful! Say it 10 times if you can say it even once! Monsieur Dolomieu, like any good geologist, liked to poke around interesting rocky areas. And so, one day in the late 18th century he was hiking in the Tyrolian Alps in Italy when he came upon a rock layer that resembled limestone, but was somehow different. Naturally, the good geologist carried hydrochloric acid with him, dabbed a bit of it onto the rock, expecting it to bubble if it was a form of limestone. But it did not! It did not effervesce! It was not limestone! Imagine the Frenchman whooping it up in the Italian Alps- he had discovered a new type of sedimentary rock. Subsequently, it was named after him and carries his name to this day as does the well known mountain range in Italy called The Dolomites. This Dolomieu fellow led quite a life- one of 11 children, at age 18 he was involved in a duel and killed a fellow member of the Knights of Malta, for which he was sentenced to life in prison but was released in one year after intercession by the Pope. Later he narrowly escaped the guillotine, was imprisoned again as a prisoner of war along with the father of Alexander Dumas, the author of The Count of Monte Christo, only to be freed once again, this time by Napoleon. Along the way he found time to study geology during his rather short life of 51 years and ventured into the Alps where he described dolomite. But Carl Linnaeus, yes the famous Carl Linnaeus of taxonomy, had already shown that this rock did not effervesce with hydrochloric acid. Yet, somehow Dolomieu prevailed and his name was pinned to this rock... otherwise we might be calling those mountains in Italy the Linnaemites!

We can also thank dolostone for keeping the atmosphere of our planet habitable over long stretches of time. The role of trees and other plants in removing carbon dioxide from the atmosphere is well known. There have been efforts to boost this process by planting a trillion trees. The soil is also a great repository for carbon and regenerative farming helps reincorporate carbon into the soil. But, Bjornerud explains, this is a short term (geologically speaking) solution. The total mass of carbon stored in plants is 500 gigatons which "turns over" in about 5 years on average (obviously much longer for trees). For soil, 2,500 gigatons are stored with an average turn over or residence time of decades to centuries. To put this into perspective, about 2000 gigatons have been emitted by human activity over the last few centuries and currently about 40 gigatons are emitted each year. Dolostone, however, plays a more prominent role in the global carbon cycle, but on a much longer time scale. Ultimately we can thank these rocks for storing 100,000,000 gigatons of near surface carbon on Earth. Wow! That's 200,000 times more than plants and 40,000 more than the soil! And that carbon stays locked away (the residence time) in dolostone for tens of millions of years, making these rocks a remarkably stabilizing factor for the climate. Bjornerud describes how this all happens in detail- CO2 enters the atmosphere primarily from volcanic activity. This CO2, when combined with water vapor, is slightly acidic and dissolves calcium from some types of rock. This calcium is carried to the ocean where it combines with bicarbonate to form calcium carbonate, CaCO3, a process of "match-making" that is often facilitated biologically by marine organisms such as algae, corals, and shellfish. As these organisms come to the end of their lives, this calcite falls to the seafloor and forms limestone, much of which is eventually transformed to dolostone where it is locked up for eons. The beauty of this system is that the process "speeds up", again geologically speaking, when the atmosphere is hot and slows down when it is cold, keeping Earth’s atmosphere at an overall stable temperature.

Many of the buildings in La Crosse have been built out of locally sourced dolostone. The La Crosse Tribune on 4/17/32 says that the "limestone rock from Grandad and surrounding bluffs and coulees was in great demand. Hardly a building downtown of any age at all rests upon a rock foundation made from limestone of local quarries." This “limestone” should actually be called dolostone. One building many of us will recognize is the iconic State Bank of La Crosse on the corner of Main and 4th Street. Built in 1888 by Alexander McMillan initially known as the McMillan Building, the dolostone used for this beautiful building came from Grandad Bluff (Tribune 9/18/97)!

Another well known building that came from locally sourced dolostone (also likely Grandad Bluff) is Batavia Bank also on Main and built in 1888.

On a personal note, the photo below shows the foundation of my house, constructed in 1906, made of dolostone, similar to many older homes in La Crosse. I like to imagine it was quarried from Grandad Bluff!

The last company to mine dolostone from Grandad Bluff was the La Crosse Stone Company. This continued until the bluff and surrounding land were bought by local citizens led by Ellen Hixon and deeded to the City on December 24, 1912 with the stipulation that it "never be used for quarry purposes again" (Tribune 8/25/35). For a brief discussion about this, click here,

https://archives.lacrosselibrary.org/blog/an-historic-look-at-grandad-bluff/ . This link includes an iconic photo of the La Crosse Stone Company with Grandad Bluff in the bacxkground:

So, despite being a rock that Bjornerud doesn't "feel any affinity for" and for whom "even the name dolomite feels dreary, evoking the doldrums", in the Coulee Region we can celebrate it as a mysterious gift from long ago. It may be shy, but it played an enormous role in the geology and history of La Crosse!