Part 3 of 4-part series
By Jim Rosenbaum
Glacial landforms dominate Wisconsin’s landscape. They are so well preserved that the last major glacial advance is known as the Wisconsin Stage. That stage began about 32,000 years ago, and concluded about 13,000 years ago. (Luczaj, 2013). Hills, ridges, lakes, gorges, river channels and marshes represent landforms left by the ice or by rivers draining the melting ice sheet. These features are ubiquitous, making Wisconsin a virtual glacial ghost town.
Everything suggests the presence of a continental ice sheet, but the ice melted back to Ontario by 11,000 years ago, just beyond written history. One may assume that the glacier was witnessed by the Native Americans, who likely hunted large ice-age mammals. Perhaps Native American legends are based in part on events of the Ice Age.
Glacial landforms tell a coherent story of multiple advances and retreats of the ice front. Boulders, cobbles, gravel, and sand were dumped into moraine ridges or molded into drumlins. Lakes were created behind ice dammed streams, only later to fuel colossal floods as the ice dams melted. For our purposes, the debris-laden moraines are the most relevant. They largely determine the shape of the southern and central shorelines of Lake Michigan.
The Western Portion of Wisconsin’s “nested bedrock saucers”
A section of the bedrock, beginning at U.S. Highway 141 near Crivitz, Wis., and trending southeast across Green Bay, the Door County Peninsula and part of Lake Michigan. Silurian dolomite, being resistant to erosion, forms the highlands between Lake Michigan and Green Bay, thus separating the two water bodies. The vertical scale is exaggerated. Image from Dott and Attig, 2004, Roadside Geology of Wisconsin. Reprinted with permission from Mountain Press Publishing Company, Missoula, Mont.
As glaciers moved across the landscape, with rocks embedded in the base of the ice, they acted as giant rasps, accelerating the erosion of softer formations, such as the shale-rich formations underlying Lake Michigan and Green Bay (Fig. 10). The resistant Silurian dolomites that surface along the Door Peninsula partially diverted the Green Bay Lobe of the glacier from the much larger Lake Michigan Lobe.
For a complete glacial history of our region, see Dott and Attig (2004), Kelley and Ferrand (1967), and Ferrand and Kelley (1967), whose books are superbly illustrated. Based on temperature-sensitive oxygen isotopes contained in shells from various levels of ocean bottom core samples, the continental ice sheets advanced up to a dozen times, each advance composed of numerous substages. However, like waves washing across a beach, destroying the line of seaweed left stranded by previous waves, glacial advances destroyed much evidence of earlier glacial movement.
Ice depressed Earth’s mantle
The ice sheets were up to two miles thick. The incalculable weight of these continental ice sheets depressed the Earth’s crust by hundreds, if not thousands, of feet. The deep, semi-plastic upper layers of the Earth’s mantle were displaced, but later rose when the ice sheets melted. This process is known as post-glacial rebound, also called crustal rebound or isostatic rebound. Most post-glacial rebound occurred as the ice melted, or soon afterward, but it continues to the present (Fig. 11).
Post-Glacial Crustal Rebound & Collapse
The crushing weight of the massive ice sheets, some as thick as two miles, depressed the Earth’s crust by hundreds, if not thousands, of feet. When the glaciers melted, the crust rebounded, but collapsed in some distal areas. This contour map shows that isostatic changes continue to the present time. Contours indicate numbers of centimeters of elevation
change per century.
Features of glaciated landscapes
During the Pleistocene Epoch, erosion and deposition by glacial action became the dominant process that determined the shoreline configuration of Lake Michigan. Previously, erosion by rivers dominated landscape formation. However, during the Pleistocene, processes causing erosion changed. In the far northern portion of the lake, the rugged shorelines and deep lake basins suggest that glacial erosion dominated. However, the curved shorelines of the mid and southern lake regions, coupled with the relatively shallow depths, suggest that glacial deposition of rock and clay tills, especially moraines, were the dominant process that determined the lake’s configuration.
Moraines are glacially deposited ridges of rock, sand, and clay that outline a former margin of an ice sheet (Fig. 12). Whenever the ice sheet stabilized – when the melting kept pace with new ice flowing forward to the glacier’s front edge – the rock and sand residue carried forward by the ice was left stranded at the edge of the ice. The longer the ice front remained in one position, the larger the resulting moraine. Now, thousands of years after the ice has disappeared, moraines accurately show the position of the front edge of the glacier. Till is a more general term for the unsorted sand, gravel and clay deposited under or in front of the glacial ice. Till is the building material of moraines.
Features of glaciated vs non-glaciated landscapes
What is different about glacial land features, compared to the ridges and river valleys that defined our region before the Pleistocene Epoch, more than 2 million years ago?
Answer: Glacial hills, such as moraines, drumlins, eskers and kames, are deposited on top of the pre-existing bedrock landscape. Marshes and lakes are common in glaciated landscapes. Note that glacial landforms are composed of loose rocks, boulders, sand and gravel. Pleistocene glacial deposits have not been lithified into solid rock. The rugged hills and narrow valleys of southwest Wisconsin, a region of the state which was never heavily glaciated, can be contrasted to the gently rolling glaciated landscape of the remainder of Wisconsin.
Moraines form the shoreline of Lake Michigan
Glacial Flowlines & Moraines of the Great Lakes
Outlines of successive positions of ice sheet borders, overlaid on outlines of the present day Great Lakes, are presented in this illustration. Arrows indicate the direction of ice movement. Note the similarity of the ice border moraines to the gross outlines of the Great Lakes, especially Lakes Michigan, Huron and Erie. This classic graphic was compiled by Frank Leverett, F. B. Taylor, W. C. Alden, and Samuel Weidman, published in 1918 in W. C. Alden’s The Quaternary Geology of Southeastern Wisconsin, Professional Paper No. 106, U.S. Geological Survey/U.S. Department of Interior.
Glacial Deposits under Lake Michigan
This map illustrates the distribution of glacial till under Lake Michigan. Glacial till is a term for rock, sand, and clay that is either overrun and reworked by an advancing glacier, or left stranded at the margin of the melting ice. Till is the building material of moraines. The southern tills were deposited about 14,000 years ago. This map appeared in J. A. Linebeck, D. L. Gross, and R. P. Meyer, 1974, Glacial Tills Under Lake Michigan, Environmental Geology Notes Number 69, Illinois State Geological Survey. Used with permission.
The glacial deposits most relevant to the modern configuration of Lake Michigan-Huron are the Lake Border Moraine Systems (Figs. 13, 14).
Especially in the southern two thirds of Lakes Michigan and Huron, the looped moraines conform with the shoreline of the modern lakes. This is logical, when you consider how moraines are formed. Since glaciers flow by gravity, and are unable to support themselves, the ice initially flowed south down the low central axis of the pre-existing “Green Bay,” “Lake Michigan” and “Lake Huron” lowlands (Fig. 13). As the ice thickened, it flowed, and the glacier expanded further. Moraines left by the ice lobes occupying these lowlands now define the shoreline of the modern lakes.
Moraines are not as prominent along the northern shorelines of Lakes Michigan and Huron. Northern regions are often characterized by rocky shorelines and deep lake basins, an indication that glacial scouring and erosion shaped the basin’s contour.
Glaciers leave Wisconsin –– at least for now
The glacier retreated from the Two Creeks area, just north of Two Rivers, Wis., soon after 11,850 years ago. Says who? What’s the evidence? There are two moraine till deposits exposed in the low bluff along Lake Michigan at Two Creeks. Significantly, a layer of buried logs, tree stumps and pine cones (Fig.15) is found between these till units.
The Two Creeks Buried Forest
Idealized diagram of the face of the low bluff at the Lake Michigan shoreline near Two Creeks, Wis. Carbon-14 dating determined the wood of the forest bed is 11,750 years to 12,000 years old. A red till overlies the forest bed. This red till contains logs of the buried forest, so it must have been deposited by a readvance of the glacier soon after 11,750 years ago. This was the last readvance of glaciers into Wisconsin. Figure based on Robert F. Black, 1974, and Dott and Attig, 2004.
Here is the interpretation of this geologic exposure: Initially, spruce grew on the older lower moraine, after a temporary retreat of the glacier between 12,000 and 11,750 years ago exposed the lake bed, allowing vegetation to grow. But the ice sheet re-advanced, blocking the lake’s drainage. Consequently, water in ancestral Lake Michigan rose, drowning the forest. Eventually, the ice advanced further, knocking over dead trees and burying them in the upper clay moraine. The youngest carbon-14 age obtained from the buried logs is 11,850 years. The geologic/glacial relations at Two Creeks are sufficiently compelling that it was one of the first locations to be selected after World War II for the then new carbon-14 dating method. Other carbon-14 dates from buried organic remains found along shorelines farther north indicate that the ice front melted back into Ontario about 10,000 years ago.
Glacial deposits, though they are immense on a human scale, are only a loose aggregate of boulders, cobbles, gravel, sand, silt and clay. Because these deposits have not been cemented into solid rock, they are easily eroded by water and frost. They are short lived, considering the broader geologic time scale. Newer deposits from future glacial advances might even replace them
Part 1: Is Earth a geriatric case?
Part 2: The rock of ages
Part 3: Wisconsin, a virtual glacial ghost town
Part 4: Post-glacial events
Food for thought, suggested field trips, and references
Photo at top of page: Jim Rosenbaum with a map of the glacial deposits of Wisconsin at his kitchen table. Photo by Dan Patrinos
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Jim Rosenbaum holds a B.A. from Lawrence University and an M.S. from Stanford University, both degrees in geology. Born in Milwaukee, now living in Whitefish Bay, Jim’s long walks along Lake Michigan and formative years in Arizona and Colorado led to his interest in natural history. An avid sailor, he’s negotiated the coastal waters of Lake Michigan along Wisconsin’s shoreline for many years. An article he wrote on beach erosion was included in Focus on Environmental Geology (1976) and in Environmental Geology (1983), both edited by Ron Tank of Lawrence University and published by Oxford. In the 1980s, he migrated into the book business, from which he recently retired. He also holds a B.B.A. from the University of Wisconsin – Milwaukee. A member of the Wisconsin Marine Historical Society, he also belongs to the American Geophysical Union.
