Wikipedia Deep Dive

Driftless Area

13 min read

The Land the Glaciers Forgot

Imagine standing on a ridgetop in southwestern Wisconsin, looking out over a landscape of steep forested hills, deep river valleys, and limestone bluffs that drop hundreds of feet to the water below. Now imagine that everything you're seeing—every ravine, every creek bed, every weathered outcrop—has been continuously exposed to the elements for over two million years, while the land surrounding it was repeatedly buried under miles of ice.

This is the Driftless Area. It's one of the strangest geological anomalies in North America.

During the last great ice age, continental glaciers covered virtually all of what we now call the Upper Midwest. These ice sheets, sometimes more than a mile thick, bulldozed everything in their path. They scraped away topsoil, crushed bedrock, and when they finally retreated, they left behind enormous deposits of debris—silt, clay, sand, gravel, and boulders all jumbled together. Geologists call this glacial debris "drift," and you can find it across most of Wisconsin, Minnesota, Iowa, and Illinois.

But not here. Not in this roughly 24,000 square mile region that straddles the corners of four states.

Here, somehow, the ice never came.

Why the Glaciers Missed This Spot

The Driftless Area occupies a curious position on the map. It sits in southeastern Minnesota, southwestern Wisconsin, northeastern Iowa, and just barely touches the northwestern corner of Illinois. The Mississippi River runs right through it, and the region's terrain looks nothing like the flat, gently rolling farmland that characterizes most of the surrounding Midwest.

So why did the glaciers avoid it?

Several factors conspired to protect this patch of ancient landscape. To the north, the Watersmeet Dome—an uplifted region of ancient Cambrian rock sitting atop even older basalt—helped deflect ice sheets around the area. The extraordinarily deep basin of Lake Superior, combined with the rugged highlands along its southern shore (the Bayfield Peninsula, the Gogebic Range, the Porcupine Mountains), created another obstacle. These highlands acted like a dam, holding back the southward flow of ice.

To the east, the Door Peninsula—that long finger of land that separates Green Bay from Lake Michigan—provided its own barrier. The bedrock there was resistant enough to partially block the Green Bay and Lake Michigan ice lobes.

But perhaps the most fascinating explanation involves what lies beneath the surface. The Driftless Area sits on fractured, permeable bedrock. Water drains through it easily. And glaciers, it turns out, need water to move.

Think of it this way: a glacier is like an enormous frozen river, but it doesn't flow the way liquid water does. Ice is rigid. For a glacier to advance, it needs a thin film of water at its base to act as a lubricant, allowing the ice to slide over the ground. In most places, pressure from the weight of the glacier melts some of the ice at the bottom, providing that necessary lubrication.

But in the Driftless Area, all that meltwater would have simply drained away into the porous bedrock below. Without lubrication, the ice sheet essentially got stuck. It couldn't push forward into this region the way it pushed across the rest of the Midwest.

An Ancient Landscape in a Young World

The result is a landscape that looks utterly different from its surroundings. Most of the Midwest was essentially reset by the glaciers. The ice scraped the land flat, filled in valleys with debris, and created the gently undulating terrain we associate with Iowa cornfields and Wisconsin dairy farms. When the glaciers retreated, surface water had to carve entirely new stream beds through the freshly deposited drift.

None of that happened in the Driftless Area. The streams here have been following the same paths for millions of years, cutting ever deeper into the ancient rock.

The numbers are striking. The bluffs along the Mississippi River in this region rise nearly 600 feet above the water. Tributaries approaching the Mississippi drop more than 660 feet over distances of less than 20 miles. The Waukon Municipal Airport in Iowa sits at 1,281 feet above sea level; the town of Lansing, just 17 miles away on the Mississippi, lies at roughly 620 feet. That's a grade that would be remarkable in the Appalachians. In the Midwest, it's almost surreal.

The bedrock exposed in these valleys and bluffs dates to the Paleozoic Era—the age of trilobites and ancient seas, long before the dinosaurs. Some of the limestone visible along the Mississippi is approximately 510 million years old. In south-central Wisconsin, the Baraboo Range exposes even older rock: Precambrian quartzite and rhyolite, the remnants of ancient mountains that were already deeply eroded before the first complex life appeared on Earth.

A Landscape of Caves and Disappearing Rivers

The fractured limestone that helped keep the glaciers out has another consequence: the Driftless Area is riddled with caves.

Geologists call this karst topography, named after a limestone plateau in Slovenia where such features were first extensively studied. Karst landscapes form when slightly acidic rainwater slowly dissolves soluble bedrock—usually limestone or dolomite—creating an underground world of caverns, tunnels, and passages.

In the Driftless Area, you can find all the classic features of karst terrain. There are disappearing streams, where surface water suddenly vanishes into a crack in the bedrock or a sinkhole, continuing its journey underground. There are blind valleys—peculiar ravines that seem to lead nowhere, formed by streams that disappeared into the earth. There are sinkholes, sometimes forming overnight when the roof of an underground cavern collapses. And there are springs, where water that vanished upstream re-emerges, often remarkably cold even in summer.

Those cold springs are ecologically crucial. They feed streams that maintain near-constant temperatures year-round, creating ideal habitat for trout. The Driftless Area is famous among fly fishers for its cold-water streams, which support healthy populations of brook, brown, and rainbow trout even as summers grow hotter across the broader Midwest.

But karst geology has a dangerous side. Water moves quickly through fractured limestone, often traveling miles in a matter of hours. There's no natural filtration, no gradual percolation through sand and gravel. If a farmer sprays pesticides on a field that happens to sit above a fractured limestone aquifer, those chemicals can end up in someone's well or a trout stream remarkably fast. Groundwater contamination is a serious and ongoing concern throughout the region.

The River That Changed Its Mind

The Mississippi River as we know it is a relatively recent development in geological terms. And the Driftless Area preserves evidence of a time when the upper Mississippi didn't exist at all—at least not in its current form.

Recent soil borings and detailed lidar imagery of the lower Wisconsin River valley have revealed something unexpected: the river in that valley once flowed in the opposite direction. Instead of running west to meet the Mississippi, it flowed east.

This has led geologists to a remarkable hypothesis. The ancient upper Mississippi—they've dubbed it the Wyalusing River—may once have flowed eastward through what is now the Wisconsin River valley, emptying into the Great Lakes and ultimately the Saint Lawrence River. The watershed divide we take for granted, separating waters that flow to the Gulf of Mexico from those that flow to the Atlantic, may not have existed.

What changed? Probably an early glaciation, one that predates the most recent ice ages by hundreds of thousands of years. An ice sheet advancing from the east could have blocked the Wyalusing River's path to the Great Lakes, creating a vast proglacial lake in the river valley. As that lake filled, it would have eventually overtopped a ridge called the Military Ridge, near present-day Wyalusing State Park. The overflow would have carved through the ridge, draining the lake—and permanently capturing the upper Mississippi, diverting it southward toward the Gulf of Mexico.

If this hypothesis is correct, a single geological event redirected an enormous quantity of fresh water away from the North Atlantic and toward the Gulf. The implications for ocean currents and climate could have been significant.

When the Ice Melted: Catastrophic Floods

The Driftless Area may have escaped glaciation, but it didn't escape the glaciers' influence. When the ice sheets melted, they released staggering quantities of water.

At the end of the last ice age, several massive proglacial lakes formed along the margins of the retreating glaciers. These lakes were often dammed by ice itself, creating inherently unstable situations. When the ice dams failed—and they often did, catastrophically—the results were some of the largest floods in Earth's recent history.

Glacial Lake Agassiz, which covered much of present-day Manitoba, Saskatchewan, North Dakota, and Minnesota, was one of the largest lakes ever to exist on the planet. At its maximum, it may have held more water than all the world's current freshwater lakes combined. When Glacial Lake Agassiz drained, it did so through a series of outlets, and one of them—Glacial River Warren—carved a channel that the modern Minnesota River now occupies. The sheer volume of water that passed through was almost incomprehensible.

Glacial Lake Duluth, the ancestor of Lake Superior, also drained through the region when its eastern outlet was blocked by ice. Its overflow carved through the Saint Croix River valley, which lies just north of the Driftless Area proper but profoundly influenced the region's hydrology.

Glacial Lake Wisconsin, formed when ice dammed the Wisconsin River, failed catastrophically as well, sending floods down the river valley and into the Mississippi.

The evidence of these floods is written in the landscape. The deep, dramatic canyons of the region's rivers—far too large for the modest streams that now flow through them—testify to the volumes of water that once surged through. The Mississippi River trench itself contains sediment more than 300 feet deep at the confluence with the Wisconsin River, material deposited by the repeated megafloods of the ice ages.

A Different Kind of Midwest

The ecological consequences of this unique geological history are profound. The Driftless Area doesn't really fit with the rest of the Midwest.

Its flora and fauna are more closely related to the Great Lakes region and New England than to the prairies of Iowa or the flatlands of central Illinois. The steep terrain, the cold spring-fed streams, the exposed rock faces, and the deep ravines create microclimates and habitats that simply don't exist on the glaciated plains surrounding it.

Before European settlement in the nineteenth century, the vegetation was a complex mosaic. Ridgetops and dry upper slopes supported tallgrass prairie and bur oak savanna—communities maintained by fire, which swept easily across the exposed heights. Moister slopes held forests of sugar maple, basswood, and oak. Protected north-facing slopes and deep valleys harbored sugar maple and basswood without the oaks. The river bottoms featured wet prairies and floodplain forests.

This diversity existed because of the terrain. The thin soils, shallow bedrock, and desiccating winds on ridgetops made them unfavorable for trees but excellent for carrying fire. South-facing and southwest-facing slopes, baked by afternoon sun, supported what are called goat prairies—steep grasslands that somehow clung to precipitous hillsides.

Fire was the key. Natural fires, often started by lightning or deliberately set by Native Americans, maintained the prairies and savannas by killing young trees before they could establish. When European settlers arrived and began suppressing fire, the landscape started to change. Woody vegetation crept into former prairies. Forests expanded onto slopes that had been open grassland within living memory.

Today, conservationists working to preserve remnant prairies in the Driftless Area must reintroduce fire as a management tool, recreating the conditions that maintained these ecosystems for millennia.

Living in the Coulees

The people who settled this region gave it distinctive names. In Wisconsin, the southwestern part of the Driftless Area is called the Coulee Region, borrowing a French term for the narrow valleys that run between the ridges. In Minnesota, the eastern Driftless is known as the Blufflands, for obvious reasons. The entire region is sometimes called Bluff Country, or—in a nod to its ancient geology—the Paleozoic Plateau.

The terrain shaped the human geography. Towns cluster in the valleys, especially along rivers approaching their confluences with the Mississippi. Roads follow the creek bottoms or wind along ridgetops; cutting across the grain of the land is difficult and expensive. Farms perch on the relatively flat ridge tops or tuck into the narrow valley floors; the steep hillsides between are often too rugged for cultivation.

This geography creates vulnerability. Those valley towns, squeezed between steep hillsides and streams fed by water draining from hundreds of square miles of upland, are prone to catastrophic flooding. The hills that make the scenery so beautiful also concentrate runoff with brutal efficiency.

In August 2007, floods devastated Gays Mills, Wisconsin, a small town in the Kickapoo River valley. In August 2018, record-breaking floods struck again, inundating Coon Valley, La Farge, and Viola. The Kickapoo River, which has a flood stage of 13 feet, rose to 23 feet. The Wisconsin Department of Natural Resources dispatched boats to rescue stranded residents. Days later, two dams failed in Ontario, Wisconsin, sending a wall of water downstream through Readstown, Soldiers Grove, and—again—Gays Mills.

These aren't freak events. They're inherent to the landscape. The steep terrain, the narrow valleys, the incised stream channels—all the features that make the Driftless Area geologically remarkable also make it hydrologically dangerous. Major floods occur on a cycle of roughly fifty to one hundred years, which means that in any given valley town, catastrophe is a matter of when, not if.

Reading the Land

There's something profound about standing in the Driftless Area if you understand what you're looking at. This is land that has been weathering, eroding, and evolving continuously for millions of years. The glaciers that reshaped almost everything else in the region never touched it.

The rock exposed in the bluffs along the Mississippi is older than complex animal life. The drainage patterns of the streams date back to before the ice ages began. The caves beneath your feet formed through processes that have been ongoing since long before humans existed.

And yet this ancient landscape was profoundly shaped by the glaciers that avoided it. The deep gorge of the Mississippi exists because glacial meltwater carved it. The strange eastward-flowing ancestor of the upper Mississippi was rerouted by ice. The cold springs that feed the trout streams emerge from aquifers recharged during periods of glacial flooding.

The Driftless Area is a palimpsest—a manuscript written over but never quite erased. Beneath the present landscape, traces of older configurations remain legible. The rivers remember their ancient courses. The rock remembers seas that evaporated half a billion years ago. The caves remember the endless drip of slightly acidic water, dissolving limestone one molecule at a time.

It's all still happening. The streams are still cutting into the plateau. The caves are still forming. The springs are still flowing. The land the glaciers forgot is still becoming itself, as it has been for longer than we can easily imagine.