Winter 1998

[TEXT ARCHIVE WEB-PUBLISHED AUGUST 2001.
ORIGINAL PRINT PUBLICATION DATE: WINTER 1998.]

The Power of Ice: Trekking the Glacial Landscape
By Sheryl De Vore

Jagged ice chunks collide and crunch at the edge of Lake Michigan on a 10° F day. Was this what it was like 16,000 years ago, you wonder, when a huge sheet of frozen water inundated the Chicago region? How did this Siberia-like world, where it seemed neither beast nor bush could survive, pave the way for the ecosystems that today offer a home to an astounding array of globally significant flora and fauna?

Strange as it may seem, the region's biodiversity is a gift of the glaciers, says Ron Riepe, geology professor at the College of Lake County in Grayslake, Illinois. That's because the places harboring the rare and wonderful plants and animals of our region — our prairies, fens, lakes, bogs, wetlands, and savannas — are products of thousands of years of glacial action.

What is now the Chicago Wilderness region began as the rest of our world began, with the formation of the Earth's crust some 4.6 billion years ago. The bare, hilly landscape, punctuated with rivers and streams, supported no life because the necessary mix of oxygen and other gases did not yet exist. Over time, a shallow sea covered some of what is now Illinois, including the Chicago region. As life evolved, creatures such as snails, clams, trilobites, and other marine animals lived in this region about 420 million years ago.

Continents shifted and collided, forming a great landmass known as Pangaea. The Chicago region was then situated near the equator, where palm trees and giant dragonflies thrived in the tropical climate. Then came the period of dinosaurs. Though none of their fossilized remains have been discovered in the region, the Jurassic-era creatures most likely lived here, too.

Approximately one million years ago, our most recent Ice Age began. Over eons, ice sometimes one-quarter-mile thick repeatedly advanced and retreated, laying the foundation for the diverse landscape we see today.

Those times were not void of animals and plants as it might seem, especially toward the end of the last glacial advance and retreat in our region some 16,000 years ago. By then, the climate had changed to support a mosaic of tundra, taiga, and spruce forests. The Chicago Wilderness region mirrored the Upper Michigan and Canadian boreal forests of today, though many of the species that flourished then have since gone extinct. Mastodons, Arctic shrews, giant beavers, peccaries, musk oxen, boreal redback voles, and other animals lived here among spruces and poplars, within bogs and marshes.

Time passed and the climate warmed further. "What replaced the spruce forest was a rich, deciduous forest," says Ed Collins, biologist for the McHenry Conservation District.

Then, about 8,000 years ago, the climate became drier. Both the new climate and the relatively flat landscape created by glacial action fostered fire. Influenced by these periodic fires and dependent upon them, the region's dominant ecosystems evolved: our tallgrass prairies and oak woods. Other ecosystems flourished, too, in areas where glacial action caused ridges and depressions to form fens, marshes, and bogs. "All those various ecosystems that evolved are still here, except for the tundra," says Collins.

Understanding this glacial geology is no easy task, and scientists continue to debate how the various landforms — the basis of the region's ecosystems — developed. But they do agree that the glacier that once covered part of Illinois came down from a spreading center near Hudson's Bay in Canada. There snow fell in winter but did not melt in summer, turning to ice under its own weight and oozing laterally southward.

This glacier was not a flat sheet of ice moving uniformly atop a flat piece of ground, but rather an amoeba-like ribbon of three-dimensional frozen water with dips and peaks, cracks, and rivers flowing beneath. As it advanced and retreated, the ice acted "like a crazy conveyor belt," carrying unsorted boulders, pebbles, and sand, says Ralph Thornton, land manager for the Cook County Forest Preserve District. The glacier deposited these particles, which are called glacial till, and picked up new ones along the way. Running water later sorted the glacial debris, Thornton says, depositing it in various places to create Chicago lake plain clays, sand dunes, beach ridges, and gravel beds beneath fens.

While the rivers flowed and the glaciers retreated, "a silt called loess was blown on top of the till," says Ray Wiggers, a geologist from Beach Park, Illinois. The silt came from the huge muddy streams that flowed here in summer when the glaciers melted. In winter, the streams were reduced to a trickle, exposing the loose silt to be blown by strong winds. "It's this mixture of silt and glacial till that helped form the prairie soils," says Wiggers, author of Geology Underfoot in Illinois (Mountain Press, 1997).

Glacial soils

At least 600 types of soils developed from the windblown silt that overlies glacial debris in Illinois, says William E. McClain, author of Prairie Establishment and Landscaping in Illinois. One of the richest of these is the black soil like that found at Wolf Road Prairie in Westchester, considered one of the finest remaining examples of a black soil prairie in Illinois, where 400 plant species exist.

A plenitude of grasses and forbs evolved in the region's diverse soils. They developed deep root systems, sometimes 20 feet below the ground, to protect themselves from the dry, hot summers and cold, hostile winters that now characterize our climate. Over the years, the organic root mass of these prairie grasses and forbs nourished and fertilized the soil, producing what some have called "the richest soil in the world." Prairies also attracted nesting birds such as grasshopper sparrows, bobolinks, and meadowlarks, whose plumages mimic the grasses' hues.

Dry prairies

Ecologists recognize several types of dry prairies in our region, including the very rare dolomite prairie. Dolomite is a type of limestone rich in magnesium. A deluge of glacial meltwater scraped away all the glacial till in some places, exposing ancient dolomite rocks deposited under the sea some 420 million years ago.

The minerals in dolomite affect soil chemistry enough that unusual plant species, such as the federally endangered leafy prairie clover and rare slender sandwort, survive only at these sites. The Hines emerald dragonfly, which now lives only a few places on Earth, can be found at specific sites in Cook and Will Counties where dolomite prairies exist.

The sand prairies along Lake Michigan are also remnants of glacial action. "The ice sheet flowed most easily over the Lake Michigan basin because it had a bedrock of soft shale and was easily eroded," Wiggers says. Indeed, the region bears evidence of three major earlier shorelines associated with the Lake Michigan basin. In time, modern-day Lake Michigan formed. Waves sorted glacial debris, depositing tiny sand particles along the lake's edge. Winds then tossed the sand particles to create dunes. At Illinois Beach State Park in northern Lake County, plants such as a prostrate shrub called bearberry and the prickly pear cactus, whose yellow blooms brighten the dunal landscape in summer, evolved under the dry, windy dune conditions close to shore.

Westerly winds also created the high dunes at Indiana Dunes National Lakeshore. Here a dense network of rare marram grass clings to the seemingly inhospitable sand along the shoreline, spreading its rhizomes sometimes 20 feet to keep the wind and waves from killing it. In nearby interdunal ponds, the rare Fowler's toad lays its eggs. Along pond edges grow rare flowers such as the insect-eating horned bladderwort.

Farther inland, skunk cabbage and ferns thrive in moist woods, while lupine grows in open oak savannas. The lupine attracts the world's third-largest population of the federally endangered Karner blue butterfly, which feeds on this plant.

Glacial deposits also created wetlands. Once upon a time, most of the city of Chicago was one big, flat wetland complex consisting of wet prairie, sedge meadows, marshes, and some dunal ridges. Silt, separated by lake and wind action, created a gooey marshland. Signs of the area's historic wetlands are still evident in places such as the Lake Calumet area south of the city. Here, wetlands such as Indian Ridge Marsh host one of Illinois' largest documented breeding colonies of the state-endangered black-crowned night-heron.

Moraines

Meanwhile, long ridges of glacial material known as moraines formed higher ground and harbored the region's woodlands and savannas. Fire travels quickly uphill, but slows considerably on the way down. When fire reached the top of one of these ridges, it slowed, encouraging the development of wooded areas, principally on the north and eastern sides, which became populated by fire-dependent oaks. The Chicago Wilderness region contains dozens of moraines. The Valparaiso moraine, one of the largest, stretches all the way from the Wisconsin line well into Indiana.

Kettles, bogs, and fens

Rare wetlands and inland lakes exist in the region because of another glacial effect: the creation of kettles. "When a glacier retreats or melts, it does so in chunks and pieces," says geologist Wiggers. "Detached chunks of ice get buried under glacial debris." Imagine a colossal ice cube submerged under the sand at a Lake Michigan beach. When the cube melts, the sand sinks into the hole it leaves. The depression left by the melting of a glacial ice cube is known as a kettle.

Some kettles became glacial lakes now dotting Lake and McHenry Counties. Cedar Lake in Lake Villa, because it is one of the region's cleanest glacial lakes, harbors several rare plants and animals.

Efforts to protect Cedar Lake by residents such as botanist Dr. Linda Curtis and others have helped preserve its quality and purity. While canoeing several years ago on Cedar Lake, Curtis discovered the state-endangered water marigold blooming among other threatened and endangered submerged aquatic plants. Rare fish such as the black-nosed shiner also swim in the waters here. The only populations of this species in Illinois are found in clear, well-vegetated glacial lakes in the Chicago Wilderness.

Kettles sometimes don't remain mere kettles. "If the water in the kettle gets acidic," says Wiggers, "the kettle becomes a bog." Bogs become acidic because they are closed systems, where no water enters except from rain. Acid develops from the slow breakdown of dead plant material in the cold bog waters. Sphagnum moss, which thrives in bogs, also adds acidity to the substrate. The cold, acidic conditions of bogs offer few nutrients, so plants get their food in other ways than from the substrate. At a bog near Lake Defiance at Moraine Hills State Park, for instance, the pitcher plant drowns unsuspecting insects in its cup-like appendages that fill with rain water.

But a kettle may also contain a fen, another kind of living history. Remember that long before the glaciers came, Illinois contained shallow seas teeming with hard-shelled creatures. When these creatures died, their shells became the foundation for the region's dolomite bedrock. In some places, glaciers or their meltwater broke and moved this exposed bedrock. As water seeped through the dolomite, it leached out some of the calcium and magnesium. When water reached the clay or shale layer, it flowed horizontally, traveling through the substrate and creating a more alkaline soil when the water reached the surface. In time, says geology professor Riepe, the fen environment may become more acidic.

Fens harbor unusual plants such as turtlehead, a plant that attracts the Baltimore checkerspot, a rare butterfly here. "These butterflies need the turtlehead in order to survive because it's the only plant that the female lays its eggs on," says Doug Taron, exhibit coordinator at the Chicago Academy of Sciences and coordinator of the regional volunteer butterfly monitoring network. The white lady's slipper and the Ohio goldenrod are two other species that thrive in the characteristic alkaline soils of fens.

Glaciers did not create neatly isolated landscapes. Instead, different types of wetlands are scattered throughout the region, sometimes next to each other. At the aptly named Glacial Park in McHenry County, for instance, you can find an acidic bog and an alkaline fen very close to one another, says Wiggers. "A slight change in elevation, the chemistry of the soil, and other factors can mean a substantial change in vegetation," says biologist Ed Collins.

Kames

Of Glacial Park's 3,000 acres, 400 are part of the Illinois State Nature Preserve system due to the importance of the area's kettle and kame — or "basin and knob" — glacial topography. The basins are the kettles and the knobs are the kames, the region's natural high rises.

Several types of kames exist at Glacial Park and each developed from different geological forces. The rises and dips in glaciers become filled with debris when a glacial river flows on top of, within, or underneath the glacier. When the ice melts, what's left is a mound of gravelly debris. Geologists think the kame at Johnson's Mound in Kane County developed in this way. A delta kame, on the other hand, developed when debris was deposited where a glacial river formed a delta at the glacier's melting edge. The two large "camelback kames" at Glacial Park are delta kames.

Johnson's Mound and the kames at Glacial Park are visible links to our geological past. Reading the glacial landscape from the sandy Lake Michigan shoreline to the elevated kames can enhance our appreciation of this globally significant region we call the Chicago Wilderness.

Sheryl De Vore is an award-winning environmental journalist for Pioneer Press Newspapers, the Chief Editor of Meadowlark, A Journal of Illinois Birds, and Assistant Editor of Chicago Wilderness.