to 10,000 B.C.
Roots — both of plants and people — depend on the soil. So the history of DuPage, like all accounts of origins, must begin with the making of the earth itself. The ground beneath tells its story in the rocks.
The deepest layer, over 4,000 feet deep and into billions of years past, is granite. The granite expanse of the Precambrian Age is called the Canadian Shield. It extends into the northern reaches of the North American continent. Geologists have concluded that this bedrock represents roots of mountains which arose and wore away over eons of time.
An account of subsequent rising and falling of the earth’s surface, often lying below an inland sea and resulting in the sedimentary layers of the Cambrian-Ordovician period, is beyond the scope of this narrative. But the overlying Niagara dolomite, curving as it does across the northern rim of the Great Lakes, requires special comment, for this limestone is popularly referred to as the bedrock. It is significant in both the geological and biological history of northern Illinois.
To delve 200 to 400 feet below the surface is simultaneously to rewind time some 400 million years. That was the era of the Silurian Sea, of a tropical climate, of crustacean forms, crinoids, and other sea creatures whose limey (calcium carbonate) remains sank to the bottom and over millions of years accumulated to a vast bulk. As other tropical debris, mud, and ferns weighed on top, limestone was formed from the compression.
The climate was tropical because of the land’s location at that time toward the globe’s equator, with all continents joined in the one great landmass called Pangea. What was to become northern Illinois once had all the layers of the later geological epochs, such as the Mississippian coal deposits of 270 million years ago. But why does southern Illinois retain these strata and not the land below DuPage?
The difference is due to Pangea’s breaking up 200 million years ago, and the North American continent drifting in a northwesterly direction. The landmass was not only heading in that direction but also tilting upward at its northern end, while central and southern Illinois came to reside in a basin. As northern Illinois became more and more exposed above sea level, the elements eroded the surface, wearing it down to the limestone base and completely weathering away hundreds of millions of years of geological history piled on top of the dolomite.
This was the scene a relatively short one million years ago. The DuPage vicinity looked the way the Galena area does today, with its high limestone hills and cliffs. Only that “driftless” region escaped the bulldozing and valley-filling effects of the glaciers from the north.
MOUNTAINS OF ICE
The Ice Age, technically called the Pleistocene Epoch, was one in which massive mountains of ice covered the region for several million years. Mile-deep ice generated seventy-five tons of pressure per square foot, creating a heat so intense that the underside of the glacier melted, causing it to slide to the south. All was leveled and flattened in its path; at the same time clay, sand, and gravel were transported in frozen form from lands to the north. Igneous and metamorphic rocks not native to the area are called erratics. The largest known erratic in the county is the granite boulder along Big Rock Trail on the east side of the Morton Arboretum. As tall as a person and twice as deep below the soil, it was a “fellow traveler” that came with the glacier. The glacial debris is unevenly distributed, varying from a few to two hundred feet deep. There are, however, limestone outcroppings at the surface in some parts of the county.
The last glacial lobe followed the natural depression carved by its predecessors, which later became Lake Michigan, Some 100,000 years ago this final glaciation started into the area. After moving laterally as far as today’s West Chicago, the glacier began to melt, leaving a series of north-to-south ridges called moraines. This new mass consisted of clay, sand, and gravel. The moving ice acted like a conveyor belt, its flow and melt balanced, depositing drifts and building the ridges. Thus the topography of DuPage is defined by the terminal moraines from west to east, now called the West Chicago, Wheaton, Keeneyville, Roselle, Valparaiso, and Tinley respectively. As the Wisconsin glacier finally retreated beyond the county 12,000 years ago, it left a series of shorelines.
The more extensive precursor of today’s Lake Michigan is known as Lake Chicago. The glacial deposit left DuPage County with an average elevation of 750 feet, which is 152 feet higher than Chicago’s Loop, which is on the old lake bed.
Two other results of the glacial action were the formation of the rivers and pockmarking of the land’s surface. The West and East Branches of the DuPage River and Salt Creek followed the depressed areas between the moraines, where the meltwater was of such torrential force as to scour out the valleys. The gravel, the looser debris, was left strewn on the shoulders and floors of each valley. Such distribution accounts for the fact that 85 percent of the county’s immediate subsurface consists mostly of clay, with scattered patches of silt and sand on upland moraines, while 15 percent consists of gravel along rivers and streams.
This distribution also accounts for the quarries’ locations in the river valleys. For example, the Elmhurst & Chicago Stone Company’s operation in Elmhurst is along Salt Creek; the one in Green Valley, along the East Branch; and the one at Warrenville Road, along the West Branch. In places the gravel lies as much as one hundred feet deep.
Early maps also show a thousand swamps, marshes, and bogs distributed over the 338 square miles of DuPage, all products of the glacier’s carving. Likewise, bowl-shaped kettles were brought about as buried giant-sized ice cubes melted, causing the surrounding gravel to cave in. A notable example may be found in the Maple Grove Forest Preserve in Downers Grove. The distinctive cavity (called the Sugar Bowl by local people) is about a hundred feet northwest of the footbridge over St. Joseph’s Creek, off Gilbert Park.
No concluding word about the Pleistocene era would be complete without recognizing the massive, vital, and continuing effects of the glaciers on DuPage. Most water issues today are related to the primordial ice. The weight and pressure of the glacier was so great on the surface that it cracked the limestone layer beneath, thus creating the crevices which comprise much of the county’s water-bearing strata called aquifers. The shallow dolomite wells provide 60 percent of the total water used today.
The glacial clay beneath the topsoil is generally impermeable. Saturation of the soil occurs easily and is followed by quick water runoff of rainfall and frequent flooding. However, in the river valleys the gravel substratum permits direct and continuing soaking into the aquifers, thereby replenishing the water supply. The implications of this landscape feature for current water-related issues will be discussed in the last chapter. But one recent incident has illustrated the continuing effect of the glacial outwash.
On the east side of the Morton Arboretum, Meadow Lake, which is situated in the gravels of the valley, began to recede mysteriously in the early 1980s. Its water had almost disappeared from view before an explanation was found. Across Route 53, in the Valley View subdivision, the utility company was constructing a lift-pump station. In order to install a concrete housing for the station, it was necessary to drop the water table by pumping. Because the stratum beneath the surface is gravel, the water table across the whole river valley dropped, thereby lowering the level of the lake dramatically, more than a thousand feet away. Cessation of pumping restored the lake’s level.
Not the least of the glacial effects was the wind-blown loess, ground so fine as to give it nutrient value and distributed by the wind across the tundra and fir/spruce (taiga) forest which followed at the edge of the retreating ice sheet. But the richness of soil awaited the next and most distinctive process in Illinois earth making.
HEAT AND GRASS
Ordinarily the hardwood trees would have succeeded the evergreens in this area, as elsewhere, as the climate warmed. But the climatic pendulum swung so far in that direction that DuPage became as dry as Nebraska is today. This hypsithermal period lasted for several thousand years, long enough to prevent the oaks, hickories, and maples from making their natural inroads north. This aridity provided the conditions necessary for prairie grass to spread from the west.
By 4000 B.C. the climate had become increasingly temperate, approaching its present characteristics, with an average of thirty-six inches of rain a year. The DuPage vicinity became part of the “Prairie Peninsula” in northern Illinois, for it is located on the edge of the prairie from the west and of the woodlands to the east, at the boundary of moisture streaming off the Gulf of Mexico and of the prevailing westerly winds. This intersection accounts for both the volatile weather and variety of plants.
The hardwoods resumed their northward march as moisture increased and would have displaced the prairie at this time, except for the introduction of a new factor — people. Newly arriving humans burned off the prairies to flush out game and make traveling easier. Only the heat-resistant burr oak on the uplands and protected groves along the rivers could survive the flames. About 85 percent of the county was prairie, and 15 percent oak and maple at the time of the first European settlement.
The topsoil of DuPage is so rich because of the organic matter from prairie plants. Their roots go as deep as fifteen feet, although the fertility develops primarily in the top two to three feet. The floodplain soil, which had run off from the ridges above, may have accumulated to a four-foot thickness, whereas on the hills it may average only six inches. Prairie ground is mollisol, a soft, black soil produced by the humus of thick grass roots, as distinct from the alfesols, characterized by high aluminum and iron content. These are the thinner, less fertile layers formed from the decomposing leaves of hardwood trees. It takes several thousand years to develop three feet of prairie topsoil but less than a day for it to be scraped off by a bulldozer. Native plants now are represented in only 1.5 percent of the county, having been replaced by European vegetation, which tends to be weedy.
An example of the fauna of prehistoric times is on display at Wheaton College’s chemistry building. It is called the Perry Mastodon, because it was excavated in 1963 on the property of Judge Joseph Sam Perry in Glen Ellyn. Such creatures became extinct about 8,000 years ago.
Settlers found black bear, buffalo, cougars, timber wolves, soft shell turtles, and red squirrel. These animals are now extinct in a modern setting, having been replaced by rabbits and raccoons. Predatory birds have likewise increased, although 148 species of song birds may still be counted at Fullersburg Preserve. The Virginia white tail deer followed European migration from the east and are still to be found in the area’s preserves. Such was the place in which people were to come hunt, plant, and build — to put down roots of the community. That human story now commences.