Geologic Time & Ancient Environments

Freeze Frame

The Ice Age in Indiana

by Anthony Fleming

Ice Ages in the Geological Record

The Ice Age refers to the period of geologic time encompassing the past 2 to 3 million years or so when the earth's higher and mid-latitudes experienced widespread glaciation by huge, continental-scale ice sheets. Geologists also refer to this time as the Pleistocene, a formal period of geologic time that began 2 million years ago and technically ended 10,000 years ago. The Ice Age is the most recent of several periods of widespread glaciation that have affected the earth. The geologic record indicates that major episodes of glaciation occurred at least as far back as 2.4 billion years ago, when life was far less evolved than today, as well as during the Carboniferous (~300 million years ago) and Ordovician (~450-500 million years ago). Other glacial episodes of less certain magnitude are also known from the rock record. So it appears that periods of much colder climate, marked by major incursions of ice sheets into lower latitudes, are a regularly recurring feature of our world.

The Ice Age Climate

It is clear that the global climate during the Ice Age was substantially colder than the comparatively mild one we inhabit today. But scientists debate both the absolute magnitude of cooling that occurred during the Ice Age as well as how patterns of weather were affected. The best evidence seems to suggest that, globally, the average air temperature was cooler by some 6 to 12° Celsius, and that daily temperature at latitudes such as Indiana fluctuated seasonally almost as much as they do today. However, there is little doubt that global weather patterns were substantially different, causing large regions in northern and eastern Canada to receive massive amounts of snowfall and to remain below freezing most of the year. Year after year, the layers of unmelted snow continued to compact into dense ice under their own weight, forming ice caps not unlike that in Antarctica today. Ultimately, these great masses of ice began to flow outward under the stress of their own weight, and the continental ice sheets were born.

A Continental Ice Sheet

As the ice sheets began to move southward out of the Hudson Bay and Labrador regions, they encountered a rolling landscape mantled with relatively thick soil and locally dissected by large river and stream valleys—not unlike that found today in parts of southern Indiana and neighboring states to the south. Ice behaves much like water, following the course of least resistance. That is why scientists believe that ice flow was initially directed along the courses of large bedrock valley systems that had developed over tens of millions of years of stream erosion during the generally mild and wet period known as the Tertiary. Some of the largest of these valleys were thought to lie along what is now the U.S.-Canada border, and were deeply incised into relatively weak rock types like shale. As the first glaciers moved over these valleys, they began to carve the soft rock into great basins, creating the forerunners of what we know today as the Great Lakes. The orientations of these basins greatly influenced the directions and even the scale of all subsequent ice advances into Indiana.

The very first ice sheet that entered Indiana appears to have arrived sometime before 700,000 years ago and came straight from the north, out of what is now Michigan. We know this because the sediments associated with this earliest glacier are choked with fragments of coal, sandstone, and distinctive reddish claystone—all rocks that come only from the center of Michigan, and these sediments are magnetically reversed, meaning that the North and South Poles were flipped! Although the Earth's polarity has reversed countless times in the geologic record, the most recent episode occurred some 700,000 years ago, during the Ice Age.

This ice sheet moved very slowly from north to south, encountering many bedrock hills and valleys that impeded its progress. As it crossed the landscape this first glacier completely changed the surface drainage that had taken millions of years to develop, damming up river valleys into lakes, diverting stream channels to the south, and in some cases completely filling up large valleys with sediment, leaving little record of their existence. The giant Teays bedrock valley system was one such victim. This valley system was the premier drainage way in eastern North America prior to glaciation, extending from its headwaters in the western mountains of North Carolina and Virginia to the Mississippi River in western Illinois. The gorge-like valley of this river was as much as 200 to 400 feet deep and crossed north-central Indiana between about Berne (Adams County) and Lafayette (Tippecanoe County). Today, there is virtually no trace at the modern land surface of the once giant river—only a deeply buried valley on the bedrock surface defined by various wells and test borings. The glacier also mowed down forests of spruce, birch, and other northern trees that had taken up residence in Indiana in response to the colder climate. This first glacier ultimately stopped somewhere about mid-state and slowly retreated northward, back into Canada, as the climate began to warm slightly, and temporarily.

Fashions and Clothing-Industry Trends During the Ice Age

Environmental conditions near this ice sheet would have been harsh indeed! Possibly as much as a half-mile to a mile thick, the imposing ice front would have been the scene of frigid cascades of meltwater and icy lakes amidst a landscape of thick, cold mud put in motion by constant freeze-thaw activity and water. The sense of desolation would have been compounded by the constant pounding of katabatic winds—cold blasts of air that roar off of the glacier as air masses passing over the ice surface are abruptly chilled and sink rapidly. In fact, an ice sheet of this scale would have probably made its own local weather. Sudden cooling of warm, moist masses of Gulf air encountering the cold ice sheet would have created torrential summer rains and near-constant "glacier-effect" snow in winter. During midsummer, at its southern extent in central Indiana, it is likely that one could have stood next to the glacier wearing shorts and a tee-shirt in weather similar to what we would experience today on a warm day in May! But the incessant cold wind pouring off the ice would be a constant reminder of the bitter winter to come.