Glaciers in the South Central US
Most people may not picture the South Central as an area that once contained glaciers, but the northern parts—northern Kansas and Missouri—were reached by the most extreme glacial advances of the late Pleistocene (Figure 6.1), and weathering and deposition indirectly associated with glaciation have occurred in other parts of this area. 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 over the northern United States. These ice sheets scraped away and ground up whatever rock was at the surface. When the ice finally retreated, it deposited the rock and dirt it had been carrying, influencing the landscape long after the ice was gone. Because glaciers affected the South Central in ways that do not directly correspond with bedrock in its different regions (which is not to say that bedrock is irrelevant), this chapter is instead organized according to the geographic areas associated with glacial processes.
Figure 6.1: Southernmost extent of glaciation over North America, at a time when glaciers covered parts of northern Kansas and Missouri.
What is a glacier?
A glacier is a large mass of ice (usually covered by snow) that is heavy enough to flow like a very thick fluid. Glaciers form in areas where more snow accumulates than is lost each year; 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. As new snow accumulates, it buries and compresses old snow, transforming it from a fluffy mass of snowflakes into ice crystals with the appearance of wet sugar, known as firn. As this firn is buried deeper, it coalesces into a mass of hard, dense ice that is riddled with air bubbles. Much of this transformation takes place in the high part of a glacier where annual snow accumulation outpaces snow loss—a place called the accumulation zone. 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 (Figure 6.2). Once the ice becomes thick enough, it flows outward to the ablation zone, where ice is lost due to melting and calving. The boundary between these two zones, the equilibrium line, is found where annual ice accumulation equals annual ice loss. Because the altitude of this line is dependent on local temperature and precipitation, glaciologists frequently use it to assess the impact of climate change on glaciers.
Figure 6.2: As dense glacial ice piles up, a glacier is formed. The ice begins to move under its own weight and pressure.
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. Alpine glaciers may fill part of a single valley, or they may cap an entire mountain range. 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 found in the high latitude polar regions of Greenland and Antarctica where temperatures are low most of the year. Keep in mind that there must be landmasses at high latitudes for continental glaciers to occur, as by definition they cannot form over open water. While persistent sea ice can and does form, the fact that it floats prevents it from flowing as a glacier does. The glaciers that stretched over northern North America as recently as 20,000 years ago were primarily continental ice sheets.
While only the two broadest categories of glaciers are discussed here, glaciers exist in a variety of forms. Even these broadest of distinctions are not quite so clear-cut (e.g., continental glaciers often have tongues that feed into valleys, which may become alpine glaciers).
In summary, glaciers grow when it is cool enough for an 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 that reached the South Central states of Kansas and Missouri flowed from centers of accumulation far to the north (in what is now Canada), and glacial growth southward through the Midwest was more a result of this lateral flow than of direct precipitation from falling snow. By 18,000 years ago, the ice was in retreat due to a slight warming of the climate—it was not actually flowing backwards, but melting faster than it was accumulating and advancing.