Geology of the Lakeshore Nature Preserve

The geology of the Lakeshore Nature Preserve is a story of a vanished land at the edge of a long-lost subtropical sea.

Ancient sands, muds, and shell layers at the bottom of this sea slowly transformed into the sandstones, shales and limestones under the Preserve today. The Four Lakes area sits atop a classic “layer cake” of these sedimentary rocks. Cambrian to early Ordovician in age, these strata record the Wisconsin environment of 480-500 million years ago.

It is an amazing thing to consider the majestic tectonic procession of these rocks from their original location of 10 degrees South Latitude (just south of the equator). Despite the slow, inexorable motion of the North American crustal plate that brought them to their current position, these layers are almost undeformed by collisional tectonic forces. Only the gentle southward tilt of the entire sequence hints at the journey.

Without this geological underpinning of Madison, the campus and city would not exist as we know them today. To slake Madison’s thirst for millions of gallons per day of drinking water, deep wells are drilled into the subterranean aquifers in the fossilized shell beds now buried far below.

Ice on campus, hundreds of feet thick

Uplift followed by eons of tectonic quiescence allowed running water to cut deep channels into the underlying bedrock. In the Madison of 50,000 years ago, the Yahara River flowed sweetly at the bottom of a steep river valley perhaps as much as 600 feet deep. Resilient sandstone layers formed extensive ledges and spring fed streams issued from limestone caves to cascade to the river far below. Incredibly, a natural event of unimaginable scale literally wiped this entire ecosystem from the face of the Earth.

The last pulse or “Wisconsin Stage” of Pleistocene glaciation generated a massive tongue of ice known as the Green Bay Lobe. It’s difficult to imagine our campus covered by a sheet of glacial ice hundreds of feet-thick, but indeed this was the case. This massive glacier approached from the northeast and finally ground to a halt just a few miles southwest of Madison. Today, a sinuous ridge known as a terminal moraine snakes its way along between Madison and Verona, memorializing the outer edge of the last great Pleistocene continental glacier.

See a Generalized Glacial geologic map of Dane County, Wisconsin

The enormous icy conveyor belt that is a glacier dumped billions of tons of glacial till on the area. Unlike running water, ice doesn’t discriminate by size or density when transporting the stony fruits of geological processes. As the Late Pleistocene global climate began to moderate, the ice edge fluctuated back and forth across the area for several thousand years, dumping and reworking quantities of glacial debris sufficient to nearly fill the ancient river valley.

The lakes begin, the isthmus appears

As the channel filled, ice dams blocked the Yahara river, forming a large impoundment known as glacial Lake Yahara. Locally, the lake extended west of present University Bay, covering all of the low, marshy area, the University Hospital grounds, and lapped against the hills of Shorewood. Eventual failure of the great ice dam allowed water levels to recede, revealing the complete transformation from a long river valley to the series of four lakes, as we know them today.

The massive ice sheet smoothed the existing topography and redistributed its leavings as it flowed and ground its way slowly back and forth across the campus. In places, this process produced characteristic elongated and rounded hills known as drumlins. We know these today as Bascom Hill and Observatory Hill. Similarly, piles of glacial debris make up Picnic Point and the entire isthmus on which much of Madison is built.

Erratics and haystacks

Glaciers sweep boulders, cobbles, gravels and sands along, frequently for hundreds of miles and dump them into unsorted piles as the ice melts. Typically these rocks have no relationship to the bedrock of the areas in which they are deposited, and thus are referred to as “erratics.” The largest erratics are termed “haystacks.” The vast majority of boulders and cobbles you will see in the Lakeshore Preserve are very old metamorphic and igneous rocks transported by the ice from northwest Michigan and northeast Wisconsin—possibly even moved from Canada across the present Lake Superior.

Wetlands and marshes appear

Approximately 12,000 years ago, glacial ice retreated well north and the Earth entered the current interglacial stage. Vegetation quickly returned to the area, and once again, examination of the geological record gives us a window into the past.

Thousands of years of lake succession generated extensive marshlands at the margins of area lakes. Fragmentary remnants of peat deposits are all that remain of these early postglacial wetlands. Analysis of pollen grains preserved in these deposits indicates that spruce forests dominated the uplands around the lakes. Thus, postglacial Madison must have resembled the present day Taiga or Boreal forests of the interior of Alaska.

Early humans and mastodons

As the flora and fauna recolonized the land, Paleoindian hunter-gatherers moved into the area in pursuit of game. A single known Paleoindian site near University Bay documents early human presence in the Preserve approximately 10,000 years ago. In addition, the UW Geology Museum displays not only a mastodon molar recovered while dredging the Olin Turville boat launch on Lake Monona, but the skeletal remains of a mastodon and associated fluted spear point found by farm children after a spring flood near Boaz, Wisconsin.

Earthworks and effigy mounds

Photo by J.M. Rothbart.

Apart from such scattered and very rare paleoarchaeological sites in the area, we know little of the small bands that roamed the campus for thousands of years. The archaeological record improves markedly as human populations developed less nomadic lifestyles and by 2,000 years ago local peoples began to construct large-scale earthworks on the high slopes overlooking the lakes. Native American turtle and bird effigy mounds on Observatory Hill as well as linear and conical mounds along the main Picnic Point path date back over 1500 years—a testament to centuries of human cultural landscape modifications. In fact, these mounds around the Four Lakes area once were part of the highest known concentrations of such structures in the world.

Madison sandstone

Early Madisonians eagerly exploited the high outcrops of a resilient sandstone and soon quarries in what are now Hoyt and Glenwood Children’s Park supplied quantities of “Madison Sandstone” that served as a widely used building material. Madison Sandstone, a quarryman’s term, is more formally known to geologists as the Sunset Point Member of the Jordan Formation. North and South Hall, as well as Bascom Hall and many other campus landmarks are constructed of blocks cut from this stratum.

Photo by Melanie McCalmont.

Unfortunately, some portions of this sandstone layer turned out to be a relatively poor building material. Poorly cemented in spots, it contains abundant feldspar in addition to fine quartz sand. As a result, many of the blocks of early buildings on Bascom Hill are deteriorating at an alarming rate. Close examination of the blocks on the south side of Radio Hall (photo detail) reveals that many of the more seriously degraded blocks (on right in detail) have been reconstructed of cement veneer and tooled to match the original decorations (on left in detail). Click the image to see a close-up of the sandstone.

The rapid weathering of these blocks does yield one actual benefit, however. When these rocks were being deposited off the coast of a forgotten Cambrian land, all manner of burrowing sea creatures churned the ocean sediments. Weathering textures accentuate these ancient burrows and thus, these trace fossils, along with ripple marks from ancient currents, can be seen on many of the old building blocks in this area of campus. Very rarely, trilobite and brachiopod impressions occur in outcrops of this rock, but none are known from campus walls.

Only one small outcrop of these older Cambrian sandstones occurs in a small streambed at the far western end of the Lakeshore Preserve, so the best way to get a sense of the bedrock geology of the area is to examine these old campus buildings.

Death by development: Dividing Lake Ridge

Despite the extensive mound building activity still evident in the area, nothing could match the unprecedented terraforming that would accompany European settlement. Indeed, environmental changes unrivaled since the Late Pleistocene have occurred in the last 150 years, and no story better illustrates the scale of the devastation Madisonians wrought on the local environment than that of Dividing Lake Ridge.

Once the highest point in Madison, Dividing Lake Ridge was a steep glacial moraine separating the marshes surrounding Lakes Wingra and Monona. In fact, when James Doty surveyed what would become the City of Madison, he chose the top of this moraine as his vantage point as it afforded the best views of the isthmus situated between two large lakes of stunningly clear water. Historical records indicate ancient trails laced the moraine. Extensive burial grounds and multiple mound groups occurred there as well. The ridge was a favorite spot for hunters to intercept waterfowl crossing from Wingra to Monona.

Unfortunately, early Madisonians looked at the extensive fens and wild rice marshes and saw only wasted swampland in need of channeling, filling and development. Similarly, Dividing Lake Ridge appeared less a priceless geological and archaeological bonanza and more like an enormous and providentially located sand and gravel deposit. The incessant need for fill sufficient to entomb five thousand acres of wetlands in and around the growing city spawned a frenzy of quarrying and in a few short years, Dividing Lake Ridge and all its irreplaceable archaeological bounty disappeared as completely as if it had never existed at all. Only a scrap of high ground carrying a small mound group located just to the East and above Vilas Zoo, itself built on a filled wetland, hints at the splendor of this lost treasure.

Speaking of paradise lost, early European settlers arriving in the area told of water clear enough to see the bottom of fifty feet depth. The current severely degraded and algae-choked waters offer no hint of this experience and none now living can recall it.

Learn more about Dividing Lake Ridge at Historic Madison Inc. >

More development, more erosion: Yahara River channel

In conjunction with the filling of the low marshy isthmus between Lakes Mendota and Monona, Madisonians conducted a related project that continues to negatively impact the Lakeshore Preserve to this day. Originally, water flowed from Lake Mendota to Monona via broad sheet flow through the isthmus wetlands and percolated through the permeable glacial till underlying them. At some point the decision was taken to channelize the Yahara River and in so doing, construct the lock and dam at Tenney Park.

Water levels rose five feet in Lake Mendota, destabilizing the shoreline and requiring extensive relocation of thousands of glacial erratics to riprap in an attempt to slow the rampant erosion. Equally alarming, rising water caused extensive delamination of peat deposits in the Upper Yahara watershed, in some cases forming immense “floating islands” which interfered with recreational navigation of Lake Mendota. In fact, the large and ever increasing river channel seen in Cherokee Marsh today and the attendant destruction of high quality fen plant communities are a direct result of this disastrous engineering project.

Lessons learned?

We still have many lessons to learn about the proper care of this priceless and ancient landscape we have inherited. Extensive active sedimentation of University Bay causes more of the ancient wetlands to disappear beneath a burgeoning West Campus construction boom and increases erosion of the north side of Picnic Point as well.

Perhaps the very existence of this Lakeshore Nature Preserve indicates the tide has turned at long last.

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