Layered by Time
Modified from John W. Attig and Lee Clayton, “Geology of Sauk County, Wisconsin” (1990). Image courtesy Wisconsin Geological and Natural History Survey (University of Wisconsin–Madison).
The bike path through the former Badger Army Ammunition Plant, since reclaimed by the Ho-Chunk Nation and renamed Mąąwakącąk, marks a boundary much older than the extent of a recovering industrial remnant. It’s also the demarcation between ancient geologic domains. To our West, a flat plain begins to undulate with waves of high prairie grasses before giving way to a skyline defined by bedrock bluffs. To our North, the verdant Baraboo Hills rise low and sturdy upon the strength of their purple quartzite core. And just steps to our East, a small, linear ridge stands up, not much taller than we are.
It’s clear even from here that the landscape is different beyond this modest ridge. Eastward, a shovel finds not bedrock but sand, rubble, and the occasional boulder, which geologists 200 years ago called “drift.” The survey sights find not bluffs, but rolling hills, which geologists today call drumlins, eskers, and moraines. 2.5 million years ago, though, there was no distinction between the landscape across this boundary, which today separates driftless and drifted.
The cause of this discrepancy, the builder of this boundary, was a giant wall of ice—miles thick, extending across Canada and down into what we now call “the Midwest.” In cycles of 40,000 and 100,000 years, beginning about 2.5 million years ago, the ice sheet grew and then retreated, sweeping across the land. Filling in low spots with drift while scraping down the highest hills. Turning riverine bluffs into pulsing hummocks. Flowing stream channels into widened lakes.
This bike path traces part of the boundary of a temporally and spatially vast icy domain. It would once have been in the bitter shadow of a towering cliff of ice. Now, the path sits at the base of this low ridge: the North American ice sheet’s edge. An ice sheet is never without motion. It flows from its center to its border: a glob of honey dropped on a tabletop. As this ice sheet flowed, it ferried drift and debris large and small, slowly building up a long, low rocky ridge at its terminus: the terminal moraine.
This story of a boundary is a story of ice and land, but it’s also a story of sun and sky. The land is shaped by ice. The ice is shaped by temperature. The temperature is determined by the composition of the atmosphere and the intensity of sunlight. The atmospheric influence depends on the amount of carbon dioxide released by natural processes, or more recently, by burning fossil fuels. The solar forcing depends on the ever-changing shape of the Earth’s planetary orbit around the sun.
This endless, bidirectional chain of cause and effect, feedbacks, and climatic pushbacks, stretching from the astronomical to the mundane and back again, is grounded right here. Right here on this bike path, at this low, long boundary in South-Central Wisconsin. Here, temperature, the Earth’s orbit, atmospheric concentrations of greenhouse gasses, and the growth and decay of ice sheets all come down to this one line, beyond which the ice could advance no further.
In the bluffs and hills and deep river valleys that sprawl to the West and South of the Baraboo Range, we can find an outlier to the politely subdued Midwest topography. Beneath the lush deciduous canopy and the low-growing jumble of cinnamon ferns and mayapples are layers of sedimentary rock, the best part of half a billion years old, yet not even a third the age of that seemingly indestructible purple-red quartzite. Lacking the brute strength to resist erosion that the heat and pressure of metamorphism bestowed on the Baraboo quartzite, these rocks are more reticent. They show themselves grudgingly, and only because rivers have scoured down through them. Where they are undercut into bluffs and forced to reveal themselves, stacked like a photo album fallen on its side, they tell of a hundred former worlds that existed in this one place.
Bone white quartz sand restlessly prowls a tropical coastline, while dunes and shallow lagoons patiently await the evolution of land plants. Waves lap higher on the shore each day as rising water turns a beach into the floor of an oxygen-starved sea, kelly green glauconite crystals the only visitors to this fetid wasteland. Each new geologic day brings a change in sea level, and with it a change in landscape. Worms burrow blindly through tidal sand. Algae grows, layer by layer, into mounds that dot the crystal-clear water in lagoons and bays. Another day, and the seawater laps the highest points on the Baraboo Range. As quickly as it submerges, it is buried in lime mud on the bed of a vast marine shelf. Only relentless erosion will show the Baraboo quartzite’s mettle, exposing it again as the soft-rock record of an unknown number of these former worlds is dismantled and carried away as sand and silt and clay, to distant lands being formed anew.
Zoom backward in time as erosion carves the land, to not hundreds of millions of years ago, but rather to mere millions, and see how the continent has changed. Mountains that dwarf the Baraboo Range have risen far over the horizon, the sea has receded hundreds of miles away, and multiple generations of rivers take their turn flowing across what is now the mid-continent. Pockets of gravel, most no larger than a suburban yard, are hidden like Easter eggs forgotten in tall grass. They are the remnants of rivers that carried material from the newly-minted Rocky Mountains far to the West. The great, great grandchildren of those rivers carve into the bedrock, establishing the first semblance of the valleys we know today. But rivers have their own mind! These flow to the east, hurrying to the North Atlantic Ocean. Only the great glaciers, rank newcomers among geologic processes here, can convince them to change their course. And almost grudgingly, just a blink of an eye ago, the water is coerced to flow South and form the Mississippi River, an ever permanent landmark to our human mind.
One and a half billion years ago, in a time that will be called the Proterozoic, on a beach near the place that will be named Baraboo, the sun rises and sets. Waves wash in and out, tides ebb and flow. The landscape is subdued. Erosion has patiently dismantled a chain of volcanoes that had loomed, with seeming permanence, on the horizon a hundred million years earlier.
Rivers ceaselessly convey sand from distant hills to the shore. With no vegetation persuading them into distinct channels—-land plants will not appear for another billion years—streams claim the entire terrain, twining and plaiting themselves again and again on their way to the coast.
Waves break and tides turn, imprinting the rivers’ latest delivery of sand with undulating ripple marks. Almost all the sand grains are tiny orbs of pure quartz, the stubborn residuum of deeply weathered rocks from barren lands upstream. After much tumbling, these quartz grains have forgotten their origins. But scattered amidst the mass of amnesiac quartz are rare crystals of zircon, chroniclers of lost landscapes. They recall not only the just-erased volcanic range, but also more ancient mountains in the remote heart of the continent.
For now, these stories will be archived underground. The sands become buried by other sediments, sealed away from the sounds of the beach. In the subsurface, grains that were once rambling vagabonds can no longer move. Formerly awash in surf, they are now wetted only by seeping pore waters, carrying just enough iron and oxygen to tint them rose pink. Jammed tightly together, the grains confederate silently into solid rock.
The seas ebb and time flows. Seismic murmurs ripple through the crust, signaling tectonic unrest. A land mass that was once a speck across the wide ocean has crept inexorably closer and is now pushing with insistence against the edge of the continent. The layer of roseate stone, the sandy remains of older worlds, is reawakened. Now a strong beam of imperial purple quartzite, it pushes back—until finally it buckles spectacularly, wrinkling the crust into a new mountain range.
Rivers immediately set to their obsessive work of flattening the undulating topography. The quartzite mountains, however, prove unusually robust. For a billion years, rivers and allied agents of erosion scour and scrub the land, but they cannot rub out these hills.
* * *
A half billion years ago, in a time that will be called the Cambrian, on a beach near the place that will be named Baraboo, waves wash out and in, and tides flow and ebb. Every few decades, the routine is interrupted by violent tempests. Surf crashes against sea cliffs of purple quartzite, the stalwart remnants of ancient mountains. The cliffs shed cobbles and boulders into sand, accumulating at their bases but standing resolute through storm after storm. Finally, even the hurricanes admit defeat and simply bury the obstinate cliffs in recycled sand—tiny orbs of amnesiac quartz, interspersed with archivist zircons that try to keep track of how many landscapes they’ve seen come and go.
Soon, plants green the land and temper the most unruly habits of rivers. But after five hundred million years and recent assistance from glacial ice, even these tamer rivers manage to carve away the Cambrian sandstones, revealing a forgotten mountain belt of folded purple quartzite whose beds were rippled by waves on a beach a billion and a half years ago, and now bask in the sun once more.
