Landforms Tell The Story
As you explore the trails and parks of the Illinois and Michigan Canal National
Heritage Corridor, you'll become aware of the beauty and variety of the
landscape along the canal route. But you may not realize that the distinctively
shaped hills and ridges, the bedrock gorges, the marshes, and the lakes
are all evidence of the activity of glaciers and glacial meltwaters.
Continental glaciers invaded Illinois
repeatedly during the Ice Age, a span of time from about 2.4 million years to
10,000 years ago. The modern landscape you now see records the retreat of the
last major ice sheet that extended into Illinois from 25,000 to 14,000 years
ago. This invasion took place during the most recent or Wisconsinan Glaciation,
which geologists estimated extended from 75,000 to 10,000 years B.P. (before
present time). During that time, the Laurentide Ice Sheet covered much of Canada
and the northern United States. Nurtured by a continental climate colder than
today's, the ice sheet grew as snow accumulated and the pressure of its own
weight caused it to change to ice and spread outward from its Canadian center.
The tongue of the ice sheet that flowed into Illinois came from the north. It became known as the Lake Michigan Lobe because it flowed as a river of ice through the Lake Michigan Basin before it spread out into central Illinois. When the glacier reached its southernmost limit about 20,000 years ago, the ice was a mile thick at Chicago--an enormous weight that depressed the land beneath. Later as the ice retreated and the glacier's weight was released, the Earth's crust began to rebound. The crust is still rebounding today, especially from Milwaukee northward. The picture on the right shows Wisconsinan end Moraines in Illinois.
The following two diagrams show what scientists think the landscape of northeastern Illinois looked like 1) about 14,000 years ago when the Wisconsinan glacier had retreated to near the present shoreline of Lake Michigan, and 2) about 200 years ago before man had modified the natural drainage left after the glacier had disappeared.
When glaciers extended into Illinois, the climate was much different from today. At the ice margin, long-haired mastodons browsed among spruce forests. The average yearly temperatures were near or just above freezing, and most of the year's precipitation was in the form of snow. In the summer, enormous volumes of meltwater and sediment flowed away from the glacier, but during winter, river volumes were reduced to a relative trickle.
Landforms to Discover
Bullfrog Lake is a kettle by the melting of a huge chunk of stranded ice from the last glacier. Kettle lakes are numerous in the morainal upland of Mt. Forest Island, which is today part of the Cook County Forest Preserve District.
Split Rock, located near the mouth of the Pecumsaugan Creek, midway between La Salle and Utica, is a picturesque area. The canal right-of-way was cut through this rocky promontry. In this area, the St. Peter Sandstone and Shakopee Dolomite are exposed along the northern side of the canal.
Starved Rock is an erosional remnant of St. Peter Sandstone that forms a high terrace in the Illinois Valley. the rock is an historic site of a successful siege against the Illini Indians, resulting in the starvation and demise of the entire tribe.
Radiocarbon Dates Set the Timetable
Although we can read the story of the events of the last glaciation from the landforms we see today, we have to probe deeper to understand the timing. By looking at the sequences of rocks and unconsolidated deposits, we can begin to understand the relative timing of geologic events. For example, where lake clay underlies glacial till, it is clear that a lake existed before a glacier advanced across the landscape. If we find undisturbed beach sand overlying glacial till, we know that a lake formed after the glacier retreated. But unless we can date the deposits, we do not know the exact chronology fo these events.
The development of radiometric age-dating techniques in the past 30 years has made it possible to establish the actual timing of certain geologic events. These techniques are based on the fact that over long periods of time radioactive elements break down (decay) at a constant rate. All living things contain the radioactive element Carbon-14, and while they are living it is exchanged freely with the carbon in the earth's atmoshphere. Once the living thing dies, however, this exchange stops; no more Carbon-14 is picked up from the atmosphere and the decay process begins. By carefully measuring the amount of Carbon-14 remaining in certain materials such as wood, peat, shell, and the bone, scientists can calculate radiocarbon ages for these materials back to several tens of thousands of years.
Radiometric dating of organic material that has been found in glacial and lake deposits in Ilinois has permitted the development of a timetable for glacial and lake events that happened during the past 25,000 years. Dates from samples of wood found beneath and within late Wisconsinan glacial deposits indicate that trees were growing in northern Illinois about 25,000 years ago when the glacier was advancing. Radiocarbon dates also reveal that the glacier reached its southernmost position about 20,000 years ago.
Radiocarbon dates tell us when the glacier retreated into the Lake Michigan Basin and that preglacial Lake Chicago formed more than 13,000 years ago. Radiocarbon ages of wood found in three abandoned beaches at the south end of Lake Michigan have also been determined. The Glenwood Beach, 60 feet above present Lake level, formed between 11,800 and 11,000 years ago. Both of these beaches formed in Lake Chicago when the glacier was still in the Lake Michigan Basin. The Toleston Beach, 20 feet above present Lake level, formed in Lake Nipissing between 6,000 and 4,000 years ago, after the glacier had completely melted. The age-dating of these beaches has made it possible to develop a detailed lake level record and deglaciation history of the Lake Michigan Basin.
Topography Dictates The Canal Route
The idea of developing a waterway to connect
the Great Lakes to the Gulf of Mexico came from the topography left after the
last glacial retreat. The history of man's use of the waterway begins with the
Indians, who used the low divide between the waters of the Chicago portage between
the Great Lakes and Illinois-Mississippi River water routes. Before the natural
drainage east of the divide was modified by man, the Chicago River flowed sluggishly
across the flat lake bed left by glacial Lake Chicago, reaching Lake Michigan
near the present site of Navy Pier. West of the divide, the Des Plaines River
flowed southward off the morainic upland onto the same flat lake bed, but its
course then turned westward through the ancient Chicago Outlet Valley to join
Illinois-Mississippi drainage route to the Gulf of Mexico. The low divide between
the Des Plaines and Chicago Rivers (known as the Chicago Portage) was only a
few miles wide. It was so flat and swampy that during wet seasons the Indians
and early explorers could travel by canoe from one river to the other without
The explorer, Jolliet, recognized the potential importance of a canal connecting the Des Plaines and Chicago Rivers as early as 1673. Recognition of the commercial value of such a canal was the impetus for moving the original boundaries of Illinois about 40 miles north to include a shore on Lake Michigan and a site for the terminus of the canal to be built when the territory would become a state in 1818. In 1830, Chicago was surveyed as the potential terminal of the Illinois and Michigan Canal. Located strategically at the mouth of the Chicago River, Chicago was at that time a small fur trading settlement and military outpost. Construction of the Illinois and Michigan Canal and its opening in 1848 were directly responsible for the growth of Chicago as a major marketing and processing center and the settlement and development of northeastern Illinois.
Prosperity, however, was not limited to the area once known as Fort Dearborn. To meet the needs of commerce along the Canal route, Lockport, Joliet, La Salle, Channahon, Morris, Seneca, and Ottawa were established. These canal towns grew with the Canal traffic, and the whole Corridor benefitted. Chicago became a manufacturing and trade center that sent building materials, machinery, grain, livestock, and many other finished materials to the populous east.