Imagine you had a thick piece of dense foam that you pushed down on with your hand. When you take your hand away, there is still an impression of it in the foam.
The impression you left will eventually "rebound" until you can no longer see that you ever compressed the foam with your hand. This is basically the same affect the thick glacier ice had on the earth's surface, although in a timeframe that covers thousands of years instead of just a few minutes.
Over time, the glacier's weight can depress the land surface a distance of approximately 30% of the ice thickness. Isostatic rebound takes place as the ice thins and retreats. It continues at an ever slower rate for thousands of years after the ice is gone. Eventually, the land surface will return to approximately the same elevation it was before glaciation. Evidence of this affect can be seen at Potawatomi State Park by the beach levels created when the Earth's surface was significantly compressed, and later when it had partially rebounded.
The glacier, and it's retreat north as it melted, also affected the water levels in the area. One of the main factors affecting the water level was the outlet through which the water flowed away. It's important to understand a little more about these water levels at various times to fully understand the significant of the beach levels created in the past.
At the present time, Port Huron, Michigan to the north, is the current outlet, and it is approximately 580 feet above sea level. The southern outlet, through the Illinois River at Chicago, is approximately 605 feet above sea level. Therefore, the southern outlet was typically a factor only when the water levels rose to at least 605 feet above sea level. NOTE: An engineering project to direct the water used by the city of Chicago away from Lake Michigan and down the Illinois River, temporarily and artificially, added the southern outlet in the recent past. This is not a factor in the isostatic rebound history and evidence seen at Potawatomi.
So let's travel back in time and fit these pieces of the puzzle (compression - rebound and water levels) together.
Before the glacier's advance to the current Potawatomi area, the land surface was probably slightly higher than the current height above sea level.
When the glacier was at its maximum advance, approximately 18,000 years ago, we can assume that the ice was also at the maximum thickness. The weight of the ice on the land surface would therefore cause the maximum compression of the earth's surface.
Between 18,000 and 12,000 years ago, the glacier was retreating and massive quantities of water from the melting ice was released. By 12,000 years ago the ice had retreated from the Potawatomi area, but, the northern outlet was still blocked by the glacial ice. Some isostatic rebounding would have occurred as the ice thinned, but there is no evidence in the area to show how much. The water level rose to approximately 605 feet above sea level until it was able to flow out through the Illinois River to the south. Since some isostatic rebounding was probably also occurring near the Illinois River, the exact level may have been slightly lower. Lakes Huron, Michigan, and Superior were all part of one larger lake referred to as Lake Algonquin. The wave cut platform, or beach level, that was created at this time is what you are standing on at site point A - Algonquin: N44 51.668 W087 24.603
Between 12,000 and 6,000 years ago, the glacier continued to retreated further north. The land at the northern end was much lower than today because it has not yet rebounded much from the thick ice cover. This opened very low outlets and the water level fell to as much as 300 feet below the present level. Lakes Heron, Michigan, and Superior became separate lakes. The evidence of this is not visible because it is below the current water level.
Between 6,000 and 5,000 years ago, enough isostatic rebounding had occurred to close the very low outlets to the north. The water level again rose to the southern outlet level of approximately 605 feet above sea level. Lakes Huron, Michigan, and Superior became one larger lake again referred to as Lake Nipissing. The wave cut platform, or beach level, that was created at this time is what you are standing on at site point B - Nipissing: N44 51.688 W087 24.637
By approximately 5,000 years ago, the current northern outlet opened and the water level dropped. Since isostatic rebound was still occurring, the outlet and therefore the water level, would have been lower than the current 580 feet above sea level. Lakes Huron, Michigan, and Superior became separate lakes again.
Between 5,000 years ago and the present, isostatic rebound continued and the northern outlet and water level gradually rose to the current 580 feet above sea level.
IATCC is the cache designation to highlight a series of EarthCaches along the Ice Age National Scenic Trail grouped into a special category called "ColdCache."
The Ice Age Trail is one of eleven National Scenic Trails designated by the National Park Service. This unique trail is entirely within the state of Wisconsin and follows along the terminal moraine of the most recent glacier which retreated about 10,000 years ago.
This project is supported by the Ice Age Trail Alliance (IATA). The goal is to bring more visitors to the trail and promote public awareness, appreciation, and understanding of Wisconsin’s glacial landscape.
The IATA has created an awards program to encourage visits to the trail and ColdCache sites. This awards program is separate from, and in addition to, any other Geo- or EarthCache awards program.
A current list of approved ColdCache sites can be found on the “IATCC Bookmark List”. More information on the Ice Age Trail Atlas, the Companion Guide, the ColdCache Project and Awards Program can be found on the “ColdCache Webpage”
The Geocache Notification Form has been submitted to Arnie L Lindauer of the Wisconsin DNR. Geocaches placed on Wisconsin Department of Natural Resource managed lands require permission by means of a notification form. Please print out a paper copy of the notification form, fill in all required information, then submit it to the land manager. The DNR Notification form and land manager information can be obtained at: www.wi-geocaching.com/hiding
Resources
Encyclopedia Britannica
Geological History of Glacial Lake Algonquin and the Upper Great Lakes; U.S. Geological Survey Bulletin 1801, Department of the Interior; by Curtis E. Larson
Geology of the Ice Age National Scenic Trail; by David M. Mickelson, Louis J. Mahler Jr., and Susan L. Simpson
Isostatic Rebound - University of Michigan GS662, 12/13/96; by Leah H. Joseph
Post-Glacial Lake Nipissing Waterworn Assemblages from the Southeastern Huron Basin Area; by Chris J. Ellis and D. Brian Deller
To Claim This Find, answer the following questions
1. If your GPS unit shows elevation, use the actual elevations to calculate the difference between site points A, B, and the southern outlet level of 605 feet above sea level. If your GPS unit does not show elevation, estimate the elevation change between site points A, B, and the current shoreline; then subtract 25 feet from the difference between site point B and the current shoreline to adjust for the change from the southern to the northern outlet.
2. Calculate the rebound rate per 1,000's of years for the time periods of: a) 12,000 to 5,000 years ago (between site points A and B); and b) 5,000 years ago to present (between site point B and the current shoreline).
3. Based on the total rebound between 12,000 years ago and present, what is the minimum thickness of the ice assuming the surface was compressed by 30% of the total thickness? NOTE: The actual maximum ice thickness cannot be calculated with the information provided because there is no visible evidence of the rebounding that occurred before 12,000 years ago and some rebounding may still be occurring today.