The dramatic topography and glacial landforms of the Kettle Moraine formed by its location pinched between the Green Bay and Lake Michigan lobes of the glaciers is rightly famous. But the glacial history of the Cedarburg Bog and the UWM Saukville Field Station area is arguably more complex and interesting. The landforms in the Bog’s area were created by multiple glacial advances over three thousand years during the most recent glacial period, the Wisconsin Glaciation.
The Wisconsin Glaciation began about 100,000 years ago, but ice did not enter Wisconsin until about 31,000 YBP (Years Before Present) and it reached its maximum extent 30,000 YBP. At that time the area of the Bog was covered by an ice sheet perhaps a mile thick. 12,000 years later (by 18,000 YBP) the glacier had melted back so that the Bog area was ice free (see the series of maps below, Figures 1-6, for the extent of glacial advances over time).
Figures 1 to 6. Series of 6 dated maps showing glacial phases, or advances and retreats of the edge of the Ice Sheet. The star shows the location of the Cedarburg Bog. Source: Wisconsin Geological and Natural History Survey, Glaciation of Wisconsin. Educational Series 36, 2011, 4th Ed. (https://wgnhs.wisc.edu/pubshare/ES036-2011.pdf)
The special time for the Field Station and Bog began 500 years later (17,500 YBP) when the Lake Michigan lobe of the glacier had readvanced to border the Bog area. By 16,600 YBP the glacier had shrunk a little and perhaps the Bog basin was just off the edge of the glacier. Over that roughly 1,000-year period, the Bog was right at, or just off, the edge of the glacier; plenty of time for a relatively shallow glacial lake in the Bog basin to accumulate fine glacial sediments.
600 years later (16,000 YBP) the glacier had melted back, completely out of the state of Wisconsin. But the Pleistocene Glaciation wasn’t done with the Bog yet. Over the next 750 years the ice readvanced so that by 15,250 YBP the edge of the glacier was once again at the Bog’s doorstep. That was the last time a glacial advance neared the Bog. Minor advances and retreats followed; and by 11,000 YBP it had disappeared from Wisconsin, never to return.
Figure 7. Map of the edges of phases of the Wisconsin Glaciation. A phase is a geologic event that represents at least a minor advance of the edge of the Ice Sheet. Each line marks the edge of the ice sheet during a phase of glaciation. Source: Wisconsin Geological and Natural History Survey, Glaciation of Wisconsin. Educational Series 36, 2011, 4th Ed. (https://wgnhs.wisc.edu/pubshare/ES036-2011.pdf).
Figure 7 shows the various phases of the Wisconsin Glaciation and emphasizes what a special and extremely active place the Bog and Field Station area was during the last ice age. I can picture huge chunks of ice breaking off of the massive edge of the glacier and getting buried by debris to form the extreme kettle-hole topography of the Field Station Beech Maple Woods (Figure 8). When the buried blocks of ice melted, they left behind the deep and dramatic kettle holes in the woods. It is also easy to recognize the parallel topographic features left by different glacial margins east of the Bog where the two branches of Mole Creek and the Milwaukee River flow south (Figure 9).
Figure 8. Shaded relief topographic map of the Field Station area created using LIDAR (Light Detection and Ranging which uses a pulsed laser to measure detailed topography). Source: Wisconsin DEM and Hillshade from LiDAR – Web Map. (https://data-wi-dnr.opendata.arcgis.com/maps/f2e49a42f5e14dd5845536408279da9d/explore)
Figure 9. Shaded relief topographic map of the Field Station, Cedarburg Bog, and areas to the east showing the parallel landforms of the Mole Creek branches and the Milwaukee River valley. (Same source as Figure 8)
The old periglacial and postglacial lake basin that holds the Bog has fascinating hidden topography covered by the current wetland. Discussion of the contours and characteristics of the Bog basin, including the islands in the Bog, would add too much to this (already lengthy) article, so I will cover that in the next FOCB newsletter.
