RAVINEs of the north shore
RAVINES-A PRODUCT OF THE ICE AGE AND THE GIFT OF THE GLACIERS
- Formed by the movement of glaciers and erosion
- Beginning 15,000 years ago, huge glaciers crept southward from Canada into the Chicago Region
As glaciers receded, left behind mounds of "glacial till" hundreds of feet high basically rock, sand, soil and gravel- Glacial till easily eroded by wind and water that cut through these deposits, creating long, deep, narrow and steep-walled gullies or ravines
- Ravines became natural drainage channels for surface water from tablelands above that had been planed flat by the glaciers
- Ongoing erosion from wind and water will cut further into table land, lengthening and deepening the ravines and altering walls, floors and vegetation
- Ravine system north of Chicago along Lake Michigan is rare, if not unique
- Widest part ends at Lake Michigan shore, where cold winter winds are funneled up and into the ravines
- Winds carry moisture that is trapped by the steep slopes and pockets in the sidewalls
- Separate microclimate from tablelands
- Temperature usually ten degrees cooler than tablelands-and much shadier
- Far fewer fires than on table lands, due to moisture and shade
One result: relic plant species, carried down from the north by the glaciers, have been able to survive and thrive with other species nourished by this microclimate- Highly unusual to see such relic species so far south-demonstrates the travels of the glaciers from long ago
- Ravines feed into and are a critical part of the drainage system of the Great Lakes, the world's largest fresh water source, representing 20% of earth's fresh water
THE STORY OF RAVINES
Ice built much of the landscape of the Chicago Wilderness. Major construction began about 26,000 years ago and ended about 13,000 years later when the glaciers finally receded from the Chicago region. Even after the ice had gone, the effects of glaciation repeatedly remade the shoreline of Lake Michigan. Ravines were formed both by the movement of glaciers and by erosion.
Just north of Chicago, along the shore of Lake Michigan, there is a good example of a ravine eco system. About 26,000 years ago, the Wisconsin Glacier formed in Canada and moved fairly swiftly southward for thousands of years, sliding on a slurry of water and mud. As this 600 to 2000 foot thick glacier receded, it left deposits of rock, clay, sand, soil, and gravel. Mounds of this "glacial till" grew as high as 200 feet creating the moraine crest what we think of as mostly flat or gently rolling or ridged surface of Northern Illinois. Its composition was easily eroded by wind and water. As the moraine crest eroded, long, deep, narrow, and steep walled gullies were cut forming the ravines. The ravines became natural drainage channels for the surface water from the moraine crest left by the glaciers.
Most ravine erosion occurs in the early spring after the soil has been broken up by the expansion and contraction of winter freeze/thaw cycles when the soil is very wet with melted snow and rain. The velocity of the water increases as it travels down the steep slopes eroding the surface and sweeping away everything loose. The water logged walls of the ravine sometimes weaken such that large chunks fall away, called "slumping", sending organic and non-organic material along the way to the lake. Soil creep and earth flow also have an effect. As wind and water erosion continues, ravines will continue to cut into the uplands lengthening the crevices and altering their walls, floor, and vegetation.
The widest part of the ravines end at the shore of Lake Michigan where cold winter winds from the lake are funneled up and into the ravines. These winds carry large amounts of moisture that is trapped by the steep slopes and pockets. Inside the ravine, the temperature is usually more than ten degrees cooler and it is also a shadier place where natural fires were infrequent compared to the upland above. Thus in the ravines, we find a very different environment from the woodlands and prairies above, and it has enabled some unusual, relic plant species, indigenous to Northern climates to remain and to continue to grow. This unique microclimate supports these specialized plants that are rarely seen this far south.
These ravines feed into the Great Lakes, the largest fresh water source in the world. The Great Lakes (Superior, Michigan, Ontario, Erie, and Huron) hold 20% of the earth's fresh water, a remnant of ancient glaciers. However, the historical evidence of the glaciers does not stop at the lakes and shoreline. It continues up through the ravines to the woodlands and forests above and the majestic prairies to the west.
THE COUNCIL RING
The Council ring in our garden derives from gathering places used by indigenous American people. Viewed as a symbol of democracy, the council ring was intended as a gathering place where all people would be equal. And, it was believed to encourage the companionship of humanity and nature. The ideal was for unity to emerge from council ring gatherings; and that unity was symbolized in the figure of the circle, the strongest and most perfect geometric form. Council rings can been seen in American gardens today, especially those designed by Jens Jensen, one of America's greatest landscape designers and conservationists. Interpreted by Jensen, council rings were simple circular stone benches with fire pits in the center. They were the only architectural element that Jensen repeated again and again in his designs.