Major chronostratigraphic and geochronologic units from Divisions of Geologic Time (modified from U.S. Geological Survey Geologic Names Committee, 2010).
The terms "Ice Age" and "Pleistocene" are often used synonymously to describe the recent period of continental glaciation that so greatly affected the landscape of Indiana and the rest of temperate North America. Although they broadly overlap, these terms have somewhat different meanings to a geologist. The Pleistocene is an epoch of geological time that began 2.6 million years ago and ended 11,700 years ago (fig. 1). Like all geologic time periods, it is defined by fossil evidence, but also by climatological indicators found in the rock record and in ice cores, which point towards a globally cooler and wetter period. The Pleistocene makes up the great majority of the Quaternary Period, which also includes the Holocene (or Recent), namely, the last 11,700 years since the Pleistocene ended.
The term "Ice Age," on the other hand, refers to a period of large-scale glaciation — an "Icehouse" climate, in other words — in which large parts of the northern (and/or southern) continental interiors were covered by ice sheets. There have been several major ice ages throughout geological time. They can be recognized in the rock record at least as far back as 2 billion years ago, and have recurred periodically ever since. For example, evidence of major glaciations is present in rocks of the Ordovician , Silurian , and Carboniferous Systems at certain places on the globe, though not everywhere, because of the phenomenon of plate tectonics, which causes the positions of continents to change through time relative to latitude and longitude. Thus, no evidence of any of these Paleozoic glaciations is recognized in rocks of corresponding ages in Indiana, because this part of North America was close to the equator when the rocks were deposited.
The limited distribution of ancient glacial deposits simply points to the fact that ice ages tended to affect the polar and temperate latitudes at any given time, while mid- and tropical latitudes were largely unaffected by continental glaciation. Perhaps somewhat ironically, glacial boulders left in Indiana during the Pleistocene Ice Age contain direct evidence of a much older period of glaciation, in the form of 2 billion-year-old rocks known as the "Gowganda Formation" and "Lorrain Formation" (fig. 2). These rocks crop out north of Lake Huron and in the Upper Peninsula of Michigan and represent the metamorphosed equivalents of the glacial till and outwash we see today at the modern land surface. They were deposited during the vast Precambrian Era and mark one of the oldest known periods of glaciation on Earth.
In summary, the Pleistocene Ice Age is just the most recent of a series of ice ages that can be recognized in the geological record. Here, we will continue to use the terms "Pleistocene" and "Ice Age" (capitalized) interchangeably, because it is this most recent period of glaciation that is so prominent in our landscape and in the popular mind.
On the global geologic time scale, the Pleistocene Ice Age began about 2.6 million years ago. There is much evidence that the climate had started cooling long before we have direct evidence of glacial deposits on the North American continent, however. This may reflect a feedback process that required a long period of geological time for widespread global cooling to take hold and for continental ice sheets to develop and move outwards into temperate latitudes from high-latitude ice caps-or it may manifest something much simpler, namely, that glacial deposits laid down earlier in the Pleistocene were removed by erosion.
The Gowganda Formation (right) and Lorrain Formation (left), 2 billion-year-old metamorphic rocks of glacial origin, transported to Indiana as glacial erratics. Photo by A. H. Fleming.
The latter process is entirely plausible, because the Ice Age is interpreted to have consisted of as many as 11 major glacial stages, punctuated by long ice-free intervals, similar to the present period we are in today, when the climate warmed considerably and erosion occurred (fig. 3). Moreover, continental glaciers are major agents of erosion, capable of quickly stripping off tens or hundreds of feet of rock and sediment either through the direct action of the ice, or via extremely large flows of meltwater produced in front of each ice sheet. Hence, as each successive glacier advanced over a previously glaciated region, there was a natural tendency to erode older glacial deposits away. This can be proven to be the case in Indiana and adjacent states, where the oldest glacial deposits from the Ice Age are preserved mainly in places protected from glacial erosion, such as in deep bedrock valleys and in front of the margins of younger glaciers. In other words, there are many unconformities within the Pleistocene glacial deposits of Indiana, in which large parts of the older glacial record are missing.
Diagram showing the timing of late Pleistocene glacial and interglacial stages based on the ice-core record from the Greenland ice cap. The relative concentration of CO2 in bubbles entrained within the ice is correlated with periods of relative cooling and relative warmth, and can be correlated with the oxygen-isotope record from the same ice cores to establish the age scale. These data show that the history of glacial and interglacial stages during the late Pleistocene is far more involved than had previously been inferred from the land record, and call into question the traditional four glacial stages originally assigned to North American glacial deposits, and the regional names applied to them. The Wisconsin and Illinoian Stages can still be recognized from the ice core data, but older stages appear to be much more complex, thus the original names "Kansan" and Nebraskan" have been abandoned in favor of the less specific term "Pre-Illinoian" to describe all glaciations before the Illinoian Stage. The entire period encompassed by the diagram is represented by glacial deposits in Marion County. Diagram courtesy of Wikimedia Commons (2011).
U.S. Geological Survey Geologic Names Committee, 2010, Divisions of geologic time — major chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2010-3059, 2 p.
Wikimedia Commons, 2011, Glacial period: Wikipedia Web page, http://en.wikipedia.org/wiki/Glacial_period, date accessed, August 17, 2011.