Glacial Landscapes

The interaction of the glaciers with the landscape is a complex process. Scouring abrades bedrock and removes sediment, while melting causes the ice to deposit sediment. Glacial features like moraines, drumlins, and kettles occasionally break the pattern of gently rolling hills found in most of the Midwest. Even in southernmost Illinois, Indiana, Ohio, and the Driftless Area where the glaciers did not reach, glacial runoff changed the landscape: meltwater loaded with abrasive sediment carved the landscape, making it more rugged.

Erosion

Thousands of years of scraping by ice can have dramatic, and sometimes dramatically varied, effects on a landscape. An important factor determining the effect is the kind of rock being eroded. Harder bedrock will be scratched and polished by sediment stuck in the ice, while frost wedging, when water freezes and expands in cracks, can eventually break chunks of rock away. Softer bedrock is much more easily carved and crushed.

The flowing ice cracks and breaks rock as it passes over, pieces of which become incorporated in the sheet or bulldozed forward, in front of the glacier’s margin. The process of glaciers picking up sediment in this way is called plucking. The less resistant rock over which glaciers move is often eroded and ground-up into very fine sand and clay (called rock flour). More resistant igneous and metamorphic rock is often polished and scratched by the grinding action of the sediments in the glacial ice. Streams of meltwater from the glacier, frequently gushing and full of sediment, cause significant amounts of scour as well. The abrasive sediments in the flowing water create potholes in the bedrock and plunge pools at the base of waterfalls. At the edge of the sheet, where the ice at last succumbs to melting, the rock is finally deposited. Piles of this rock form many of the distinctive landforms found in the Midwest today.

How do we know the mark of glaciers?

How do we know that striations, polish, scoured basins, U-shaped valleys, and the variety of deposits attributed to glaciers are in fact a result of glacial action? Before the modern understanding of the ice ages, many believed that the features now attributed to glaciers were the result of a great flood similar to the one found in the Biblical story of Noah and the Ark. By studying modern glaciers, however, geologists have come to understand the resulting features of glacial scour and deposition that are readily identified in much of the Midwest. Modern glaciers include the large-scale ice sheets in Greenland and Antarctica as well as the small-scale valley glaciers found in mountain ranges in places such as Alaska, Canada, and the Alps.

The nature of the glacier causing the erosion is also crucial. Because continental glaciers spread from a central accumulation zone, they can’t go around peaks in their path, so they instead slowly crush and scrape them away. For the most part, this results in flatter landscapes. Conversely, alpine glaciers tend to follow the existing topography, flowing downhill. This frequently causes them to scour existing low places, making them lower still. While this gouging increases the overall relief of an area, anything directly in the path of the ice is flattened. For example, a glacier might deepen a valley while surrounding peaks remain high, yet the valley itself, initially cut by a narrow stream into a sharp V-shape, is smoothed into a distinctive U-shape by the wider glacier.

Deposition

Figure 6.2: Glacial features.

Figure 6.2: Glacial features.

As glaciers scrape over the earth, sediment is incorporated into or shoved ahead of the advancing ice (Figure 6.2). The unsorted mixture of boulders, gravel, sand, silt, and clay that is picked up and later deposited by glaciers is called till. It is important to note that whether a glacier is advancing, in equilibrium, or retreating, its ice is still flowing forward, like a conveyor belt that is constantly depositing till at its margin. Where a glacier stopped its advance and then melted back, a ridge of till that had been pushed in front of it is left behind, marking the farthest extent of its margin, or terminus. A ridge of till formed this way is called a moraine, and it may range in length from hundreds to thousands of meters. A drumlin is a teardrop-shaped hill of till that was trapped beneath a glacier and streamlined in the direction of the flow of the ice moving over it. The elongation of a drumlin is an excellent clue to the direction of flow during an ice sheet’s most recent advance.

Meltwater flowing off a glacier also leaves behind deposits. Unlike till deposits, meltwater deposits are well-sorted: large rocks can only be moved by high-energy water, while finer sand and mud are washed downstream until enough energy is lost that even they are dropped. In other words, the faster the water is moving, the coarser the sediment deposited (Figure 6.3). As a glacier melts, streams of sediment-laden meltwater often create networks of braided streams in front of the glacier. Streams of meltwater flowing under a glacier can deposit sand and gravel. When an ice sheet retreats, these snaking ridges of stream deposits, known as eskers, are left standing.

Other glacial features include kettles, kames, and erratics. Kettles are depressions left behind by the melting glacier. Blocks of ice may be broken off from the glacier and buried or surrounded by meltwater sediments (Figure 6.4). When the ice eventually melts, the overlying sediments have no support, so they frequently collapse and form a depression that often fills with water to become a lake. Many kettle lakes and ponds are found throughout the Midwest, particularly the 10,000 lakes area of Minnesota, and most of the inland lakes of Wisconsin and Michigan formed in this way as well. Kames are formed in nearly the opposite way;; layers of sediment fill in depressions in the ice, leaving mound-like deposits of sorted sediment after the glacier retreats (Figure 6.5). Often the kettles and kames occur near one another. Erratics are rocks that the ice sheet picked up and transported further south, sometimes hundreds of miles from their origin.

Figure 6.3: Moving water deposits sediments in what is known as a horizontally sorted pattern. As the water slows down (i.e., loses energy), it deposits the larger particles first.

Figure 6.3: Moving water deposits sediments in what is known as a horizontally sorted pattern. As the water slows down (i.e., loses energy), it deposits the larger particles first.

A well-sorted deposit has a relatively uniform grain size.

Figure 6.4: Steps in the formation of a kettle lake.

Figure 6.4: Steps in the formation of a kettle lake.

Erratics are often distinctive because they are a different type of rock than the bedrock in the area to which they’ve been transported. For example, boulders and pebbles of igneous and metamorphic rocks are often found in areas where the bedrock is sedimentary; it is sometimes possible to locate the origin of an erratic if its composition and textures are highly distinctive. Anyone who has tried to till a field or garden in the Midwest is familiar with rocks like this.

Figure 6.5: Glacial sediment deposits and the resulting hills called kames.

Figure 6.5: Glacial sediment deposits and the resulting hills called kames.

Periglacial Environments

Though a few areas in the Midwest were never covered by the ice sheet, the entire area felt its effects. The portion covered by the ice sheet was scoured and covered with glacial deposits; the area south of the ice sheet has its own distinctive landscape and features because it was next to the ice margin. This unglaciated but still affected zone is called a periglacial zone.

The average annual air temperature in a periglacial area is between -12° and 3°C (10° and 37°F). Though the surface of the ground may melt in the summer, it refreezes in the winter.

There are a variety of features associated with a periglacial zone that also provide clues to the extent of the most recent ice sheet. In the tundra-like environment of a periglacial zone, aeolian, or windblown deposits are common. Sand dunes and wind-transported sediments are found in former periglacial areas and in glacial lake bottoms of the Midwest.

The permafrost associated with the periglacial area, in which the ground is frozen much of the year, can cause mass movement of sediment. When the surface layer of the permafrost ground thaws, it is full of moisture. This water-heavy layer of soil may move rapidly down a hill in a process called solifluction.

Physical weathering is the break-up of rock due to physical processes (such as erosion by wind, water, and ice) rather than chemical processes.

Physical weathering of the bedrock is magnified in the periglacial environment because of the freeze-thaw cycles associated with permafrost. When water enters the cracks and fissures in the ground and subsequently freezes, the ice wedges the cracks farther and farther apart (Figure 6.6). Freeze-thaw is important in any climate that vacillates above and below the freezing point of water. Because ice takes up more space than water, the pre-existing cracks and fractures are widened when the water freezes. Along ridges, rocks are eventually broken off as ice wedges continue to expand in joints and fractures. The boulders and blocks of bedrock roll downhill and are deposited along the slope or as fields of talus. Frost action also brings cobbles and pebbles to the surface to form nets, circles, polygons, and garlands of rocks. These unusual patterns of sorted rock are known as patterned ground. Solifluction and ice wedging are found exclusively where the ground remains perennially frozen, yet is not insulated by an ice sheet. Such conditions only occur in areas adjacent to ice sheets. While conditions like these existed in the Midwest at this time and led to the formation of patterned ground, any evidence was subsequently covered with glacial sediment or eroded away.

Figure 6.6: Physical weathering from a freeze-thaw cycle.

Figure 6.6: Physical weathering from a freeze-thaw cycle.