Glaciers of the Midwestern US
The ancient geologic history of the Midwest is often disguised by its more recent geologic history, one that was dominated by glaciers. During the Quaternary period, which began just 2.6 million years ago and extends to the present, ice at times extended southward from the Hudson Bay area and began to encroach on the northern United States. At different points during the Quaternary period, ice has covered all of the Midwest except for the extreme southern parts of Illinois, Indiana, Ohio, and a unique region called the Driftless Area (Figure 6.1). These ice sheets scraped away and ground up whatever rock was at the surface. When the ice finally retreated, it dropped its load of rock and dirt, forming much of the landscape we see today and obscuring the bedrock below in many feet of sediment. More than any other force, the glaciers are responsible for the landscape of the Midwest: they smoothed peaks, filled valleys, pocked the area with ponds, and carved the Great Lakes. Because the ice sheets affected the Superior Upland, Central Lowland, and Inland Basin similarly, this chapter discusses the Midwest as a whole.
Glaciers will only form under certain conditions and in specific environments. A cold climate and sufficient moisture in the air for the precipitation of snow are both necessary factors that permit at least some snow to last year-round. This allows for the build-up and compaction of snow that will eventually become glacial ice. Sufficiently cold climate conditions exist at high altitudes and high latitudes.
Glacial ice is formed as snow is buried;; the weight of more snow above causes lower layers to compact. Individual flakes break down by melting, refreezing, and bonding to the snow around them, eventually forming grains called firn. This process can be facilitated with water filling the space between flakes, but the pressure alone will cause the flakes to melt and refreeze. Air is forced up and out, or into bubbles, increasing the density of the ice. As more snow falls at the surface, adding more weight, the firn becomes denser and denser until the ice crystals interlock, effectively leaving no space between them—this is known as glacial ice. At a depth greater than about 50 meters (165 feet), the pressure is high enough for plastic flow to occur. Ice flow is driven by gravity, and it causes movement downhill and out from the center.
Most broadly, there are two types of glaciers: smaller alpine glaciers and larger continental glaciers. Found in mountainous regions, alpine glaciers have a shape and motion that is largely controlled by topography, and they naturally flow from higher to lower altitudes. Continental glaciers are much larger, and they are less controlled by the landscape, tending to flow outward from their center of accumulation.
It is not surprising that today’s continental glaciers, also called ice sheets, are located in the high latitude polar regions of Greenland and Antarctica, where temperatures are low most of the year. There must be landmasses at high latitudes for continental glaciers to occur, as they cannot form over open water. While persistent sea ice can and does form, because it floats, it does not flow as a glacier does. The glaciers that stretched over North America 20,000 years ago were primarily continental ice sheets.
Alpine glaciers are found at high altitudes, and they sometimes occur relatively close to the equator. They accumulate snow at their tops and flow downhill. Alpine glaciers may fill part of a single valley, or they may cap an entire mountain range.
While only the two broadest categories of glaciers are discussed here, glaciers exist in a diversity of forms. Even this broadest of distinctions is not completely clean-cut (e.g., continental glaciers often have tongues that feed into valleys, which may become alpine glaciers).
In general, glaciers grow when it is cool enough for the ice sheet to accumulate snow more quickly than it melts. As they grow, ice sheets become so massive that they flow outwards, covering an increasing area until melting at the margins catches up to the pace of accumulation. Glaciers in the Midwest flowed from centers of accumulation to the north (now Canada), and glacial growth southward through the Midwest was more a result of this lateral flow than of direct precipitation from falling snow.