Landscapes are histories written in layers, tangled webs of cause and effect. From the deep, miasmal past to today’s Anthropocene age in which human activity is reshaping Earth’s surface features, these layers aren’t just stacked flat, but are superimposed and intersecting. The landscape here in Wisconsin, where the web extends back more than 2 billion years, inspires and mystifies. It’s taken geologists over 200 years to piece together our landscape’s story, and that work continues unfinished today. Dedicated scientific research has revealed how climate’s repeating impacts have molded the Earth’s face over millennia, and how human activity is affecting changes over mere decades. To those listening, Wisconsin’s landscape begs the questions: What are these layers? And what forces spun them into this web?
The foundation of Wisconsin’s landscape is bedrock that ranges from the very ancient Precambrian igneous and metamorphic rocks in the north, to the Paleozoic ocean-born sandstones and limestones in the south and east. In much of the state, these rocks are hidden from view, covered by a deposit called “drift.” Understanding the nature and source of drift was one of the first strands of our landscape web that nineteenth century geologists needed to untangle.
Drift is a curious jumble of gravel of all sizes—from fine clay and sand to pebbles and boulders (often polished smooth), all mixed up and draped over the bedrock like a blanket thousands of square miles across Wisconsin and neighboring states. Geologists wondered if water had done this. In steep mountains, a swollen stream can push along boulders and easily move smaller debris like sand and gravel. But when a stream’s velocity slows, it begins to drop its load of sediments in a predictable way. Boulders drop out first, followed by pebbles on top of that, and then sand falls out and eventually silt and clay as the water becomes quiet. The rock debris is not jumbled. It is stratified or “sorted” according to the weight and size of the material. So, rock debris deposited by flowing water is orderly and layered with the biggest grains below and the smallest above. But this is not the case with drift.
Even more perplexing, geologists found drift on hilltops above the reach of any river or stream, and this drift contained rock types that were not from the local watershed and could only have come from bedrock hundreds of miles away. At first blush, this didn’t make any sense. But early naturalists developed a theory to explain these seeming impossibilities: drift was the alluvium of the biblical flood, which would have had the power to carry rocks long distances and drop them on top of submerged hills. Or, the flood swept icebergs containing rocks far from their origins and stranded them here. Still, lingering questions remained. Where had all the drift gone in the southwest of Wisconsin and other areas that would presumably have also been inundated? Why were there different sheets, or layers, of drift with different compositions, if it had all come from a single great flood?
Though biblical explanations of Earth’s phenomena still dominated thinking during the early years of European settlement in America, things began to change in the late 1700s. In 1785, Scottish geologist James Hutton proposed his “theory of uniformity”—that the geological processes at work today are the same ones that operated in the past. To understand what happened in the past, we need to look at current mechanisms of landscape erosion (such as wind, water, ice) and landscape creation (such as sedimentation and volcanoes). In 1830 another Scotsman, Charles Lyell, reiterated this concept even more forcefully, spurring new understandings in geology.
Taking this message to heart, Wisconsin geologists looked for places where a material similar to drift was still being generated. Their clue to this strand of the web lay far away, in places that bore little resemblance to the prairies and rolling hills of the upper Midwest. It was in high mountains, like those in Alaska and the European Alps, that recent drift-like material could be found, surrounding active glaciers.
Mountain glaciers tear and grind up rock on their slow descents from their originating cirques and ice fields. As the glaciers reach lower and warmer elevations, summer melting at the front edge releases the trapped rock fragments, creating a jumbled-up layer of glacial till—silt, sand, gravel and rock debris—in various shaped deposits called moraines. Most important for this story are terminal moraines—significant ridges of till that have been deposited over time along the leading edge of glaciers at the point of their farthest advance.
Geologists not only recognized that valley glacier till was similar to Wisconsin drift, they also located several terminal moraines in Wisconsin, though on a scale much larger than the deposits in an alpine valley. These moraines trace a path from the Twin Cities’ eastern suburbs across Wisconsin to Antigo in Langlade County, and then south through Madison and Janesville. If the same mechanisms were at play here that glacial geologists had observed in the high mountains, then the radical implication was that much of Wisconsin had at one point been covered by vast sheets of ice. It was an incredible revelation considering that the nearest active continental ice sheet was on remote Greenland near the Arctic Circle.
This solved the mystery of drift’s origin but was still only one strand of the landscape’s tangled web. The past presence of a continental ice sheet on North America raised more questions than it answered. When was it there? How and why did it melt? What did the ice look like and move like? These questions made it necessary to accept that our temperate midcontinent climate had not always been this way—a jarring notion at a time before we understood that the climate is always changing, today by means of anthropogenic global warming with its melting polar ice caps.
Thomas Crowder Chamberlin, one of Wisconsin and North America’s most noted geologists, described himself as “born on a moraine” in 1843 in southeastern Illinois. His family moved shortly thereafter to southern Wisconsin, near Beloit. He was inspired to become a geologist by the glacial landscape upon which he grew up. His work provided foundational insights into this landscape. His career spanned many institutions, from Beloit College to the Whitewater Normal School (now UW-Whitewater), to UW-Madison, and the University of Chicago. He was president of UW-Madison from 1887 to 1892, the first from the Midwest. He was Wisconsin’s chief geologist, the lead glacial geologist at the U.S. Geological Survey, and founder of The Journal of Geology. Through it all, he focused on his home landscape in southern Wisconsin.
One area that particularly interested him was the Kettle Moraine, a narrow region west of Milwaukee that trends northeast to southwest between West Bend, Delafield and Whitewater. Here, he figured, two lobes of the continental ice sheet had split around the high limestone outcroppings of the Door Peninsula. Further south, these lobes rejoined to create a wide swath of collapsed ice and glacial debris between them. But moraines aren’t the only glacial feature here. The kettle part of the region’s name comes from another conspicuous glacial landform: small, deep lakes called kettles by settlers after the cast iron pots of the day. Kettles form after the retreat of the main ice sheet, which leaves behind large chunks of remnant ice embedded in the postglacial tundra. These can be the size of large buildings and are covered in thick blankets of insulating sediment. The blocks can remain, melting ever so slowly, for thousands of years even after the glacier has retreated to the north. When they finally melt, they leave behind a circular depression, which often then fills with a deep, round lake—a familiar feature of Wisconsin’s landscape.
In reading the web of the Kettle Moraine landscape, a story thus unfolds: here was the edge of an ice sheet, where the margin stayed stable for long enough to build up a massive debris pile in which large blocks of ice remained for hundreds of years, even after the retreat of what Chamberlin called “the intruder.” This left behind a gravelly ridge pocked with deep, isolated kettle lakes. In this story, Chamberlin saw another strand to be untangled: the drift of Kettle Moraine and points north is substantially different in character from drift continuing to the south. If Kettle Moraine is evidence of the terminus of a continental ice sheet, what does it mean that the drift of a different sort continues past that edge?
Chamberlin believed, along with a growing consensus of geologists, that there had been not just one large ice sheet that covered northern North America, but several in a cycle of advance and retreat. They painstakingly parsed out the differences between units of drift across the Midwest, eventually identifying as many as eight major periods of glaciation. This confirmed the prevailing idea in Europe, where geologists had evidence of their own ice sheets, that the most recent geologic period was a “Great Ice Age” when glaciers repeatedly intruded to the south. For Chamberlin, this explained the “what” of his native Wisconsin landscape, but not the “why.” To answer that deeper question, it became necessary to dive into the messy, intertangled dynamics of Earth’s climate system.
When Chamberlin first approached this question, there were two main schools of thought. The first, which hasn’t passed the test of time, held that elevation was key to the inception of ice sheets. This stemmed from the observation that the only existing glaciers in North America were mountain glaciers. The high elevation kept them cool enough to be stable and even grow. So, to explain evidence of glaciers at low elevations, this group believed that the land surface of Wisconsin had once been alpine in height. The climate hadn’t changed, they held, the elevation had just plummeted.
The other prevailing idea, supported by Chamberlin and eventually proven correct, was just the opposite. The climate had been changing with temperatures cycling through cold and warm periods. When it was cold, ice advanced out of the Arctic, carrying with it the power to re-mold the landscape and bury it under drift. As is often the case in science, untangling this part of the web revealed another knot. For centuries, people had observed and recorded temperature, rainfall, and other indicators of climate. On average, they found a remarkably stable climate. So, to explain glacial cycles, scientists needed to discover what force could perturb the system enough to make temperate Wisconsin look like Antarctica.
One explanation for glacial cycles was at the time called Croll’s hypothesis. A similar theory now referred to by paleoclimatologists as Milankovitch theory holds that slight variations in Earth’s orbit change the amount of solar energy that the land surface receives. Periods of low energy coincide with ice sheet advances. Conversely, when solar energy is high, the ice melts back, leaving behind moraines, kettles, drift, and other evidence of a glacial episode.
The orbital change theory of the cause of ice ages explains much of the variation in Earth’s climate over the past 2.5 million years. Chamberlin, in 1897, identified another force that might be involved. Carbon dioxide had long been known as a greenhouse gas important to maintaining stability in Earth’s climate; Chamberlin’s insight was that its concentrations might have changed over time and in turn changed the climate system. Today, these two theories hold. Over timescales of about 10,000 or 100,000 years, climate is moderated by solar energy. In deep time, over millions of years, carbon dioxide concentrations can determine whether solar changes tip us into a hothouse or ice age climate. What Chamberlin didn’t know, and what we’re learning now firsthand, is that carbon dioxide can also change climate on timescales of decades when it’s released to the atmosphere in unnatural quantities. Inspired by his home in the Kettle Moraine, Chamberlin spent much of his career looking for the forces powerful enough to weave the web of his native landscape. The work of understanding the intricacies of how ice carved through Wisconsin, though, didn’t end with him.
If there is one Antarctic glacier you’ve heard of, it’s most likely the Thwaites Glacier. About 75,000 square miles in area, Thwaites is known among sea-level experts as the “doomsday glacier.” The ice terminus’ tenuous stability in our changing climate means there’s a high risk of collapse. If this were to happen all at once, global sea levels could rise more than two feet, just from this one glacier.
The Thwaites Glacier was officially named in 1967 for a Wisconsin man who never set foot on the Antarctic continent. Frederik Thurville Thwaites was born in 1883 in Madison, and after his college graduation in 1906 he became one of the best readers of the glacial landscape. His accomplishments came despite a tumultuous career, one in which he never earned his doctorate in geology and was long denied a faculty position at UW-Madison. He settled for a job as a curator of the University’s Geology Museum, with occasional opportunities to teach. He spent most summers in the field developing maps of glacial features in Wisconsin and he wrote one of the first textbooks on glacial geology. Thwaites’work was accurate enough that even now, 60 years after his death, geologists still turn to his maps and interpretations as a starting point for their own research.
Whereas Chamberlin was primarily focused on the causes of glaciations, Thwaites’ expertise as a field geologist was in looking at the land and mapping its web. Beyond moraines and kettles, the landscape features that Thwaites identified and mapped include drumlins, eskers, and outwash plains—all chapters in the story written by ancient ice. Drumlins are elongate, teardrop-shaped hills that are ubiquitous in southeastern Wisconsin. They form underneath ice sheets and point in the direction of ice flow. Glacial geologists use the orientation of these hills to determine where ice came from and how it moved over time.
Eskers are like river channels in reverse. When water melts on top of a glacier, it flows through holes and cracks down to the bottom of the ice sheet where it forms systems of meandering subglacial streams. Instead of incising into the ground like a river would, these subglacial streams cut upward into the ice above. Later, as the glacier melts, the sediments on the bed of this internal stream drop out on the ground as a long, winding ridge of sand and gravel. Eskers are frequently mined for these construction materials.
Outwash plains are large, flat areas covered in sand that begin at the melting edge of the ice. As debris drops from the ice all along its broad front, multiple streams and rivers flowing off the melting glacier carry the lighter sediments along, spreading them out over a wide area that eventually becomes a level plain. The city of Antigo sits on one of these vast plains, bounded by the higher terrain of a terminal moraine. To skilled landscape readers like Thwaites, these features reveal a narrative of the deep past.
Since the time of Thwaites, Wisconsin has continued its tradition of pioneering glacial and climatic research. Today, you can find these researchers in our colleges and universities, at the Wisconsin Geologic and Natural History Survey, at the Department of Natural Resources, and elsewhere. Why here? One answer is our geography. Wisconsin preserves the web of glacial landscape features because it happened to be right on the edge of the most recent advance of the intruding ice. The story is easier to read here, the web that much simpler to untangle. For this reason, the most recent period of glaciation in the cycle of ice sheet growth and retreat is called the Wisconsin Glaciation by geologists around the world.
Though these glacial features seem endlessly enduring, they are not bedrock-type formations. They are mainly loose materials vulnerable to the natural eroding forces of wind and water—and susceptible to powerful earthmoving equipment. Moraines and eskers are quarried extensively for sand and gravel. In urban areas once distinct moraines are now covered by housing and industrial developments. While there is no shortage of glacial features in Wisconsin, there is also value in protecting the most significant for the generations to come.
Chamberlin and Thwaites, and others, with a curiosity inspired by the place they called home, have helped us learn to read this story and to better understand the mysterious forces that shape the world around us. Here in Wisconsin, we live in a world shaped by ice. Features like moraines, kettles, drumlins, eskers, and outwash plains determine in part where we build our houses, develop our cities, and plant our crops. Instinctively, we adapt and nestle ourselves into the landscape’s web, into the inescapable imprint of ancient ice.