Eons before man strode the earth, global climate change irreversibly altered the topography of North America; leaving a legacy that affects life in Indiana, yet today. To understand the full measure of these global changes, we must travel back to the Pleistocene, commonly called the Ice Age. Geologist estimate that this period started 2 million years ago and ended 10,000 years ago. Variations in the earth’s orbit and axial tilt, its wobble, brought about dramatic global cooling. As the earth cooled, massive ice sheets formed – great glaciers that covered the earth’s higher and mid-latitudes.
In our latitude, seasonal daily temperatures varied much like they do today; cold winters followed by spring and summer warming. Globally, however, the average temperature was cooler than it is today (10 to 20 degrees F), and the weather patterns were much different. Nearly continuous snow fell over the northern latitudes, Canada, Scandinavia, and Siberia. Since the temperatures in these latitudes seldom rose above freezing, the snow rarely melted. It accumulated, growing deeper and deeper each year.
Snowflakes, which consist of six-sided ice crystals, are quite fluffy when they fall. Over time, however, they lose their hexagonal shapes, growing more granular and dense. The snow that survives the first melting season, it is called firn. Compressed by the ever-increasing weight of the snow above, the Pleistocene firn grew thicker, eventually becoming glacial ice. In places, the ice sheets were up to two miles thick, exerting enormous pressures on the lower ice layers [for example, under ice a mile thick, the pressure would be greater than 2,000 pounds per square inch]. Squeezed by these pressures, ice becomes plastic, deforming, and flowing slowly toward areas of less pressure. Like liquid water, amorphous ice follows the path of least resistance – downhill, outward, and away from the thicker layers of ice.
When new snowfall exceeds the rate of melting at the face, the glacier creeps forward. As it advances, it dislodges large masses of rock and soil, leveling hills, and filling in valleys.
When the rate of new snowfall is equal to the rate of melting, the glacier face appears stationary. Although the face does not advance, internally the ice continues to move, creeping from the center of the ice sheet to the edge. Like a giant conveyor belt, the ice continually scours forward large masses of soil and rock. At the face, the ice melts leaving the debris to form large moraines. The longer the leading edge is stationary, the larger the moraine becomes.
Finally, when the rate of new snowfall is less than the rate of melting, the face appears to move backward. Even while the glacier retreats, its internal ice flow never reverses; it is always from the center outward, bringing along its load of soil and rock. As the ice melts and the face recedes, the debris forms broad expanses of glacial till. In Indiana, some of this till is over 300 feet deep.
During the Pleistocene, the snow that accumulated over thousands of years formed vast continental ice sheets. These glaciers slowly, inexorably, inched southward across the width of North America. Similar ice sheets formed across Europe, Asia, and Antarctica. Only the Antarctic ice sheet remains. The volume of water sequestered in these ice sheets caused sea level to drop 300 ft. Following long-term swings in global temperature, these sheets advanced during colder periods and retreated during warmer periods. With each forward advance, rock and soil from Canada, Wisconsin, and Michigan were conveyed into Indiana. Along the way filling ancient streambeds and leveling the primordial landscape, a landscape not unlike the hills and valleys still found in southern Indiana.
At least four ice advances occurred during the Pleistocene. The most recent, the Wisconsinan, ended 10,000 years ago. At its greatest extent, it covered two-thirds of Indiana, reaching as far south as Terre Haute, Edinburg, and Richmond. Although the Wisconsian ice sheet did not reach areas of southern Indiana, they were inundated by the glacial meltwaters. Enormous volumes of water, released as the glacier retreated, caused the Wabash, Ohio and the both forks of the White rivers to flow much wider and deeper than they do today. Sand and gravel carried in these meltwaters settled to the bottom as the rivers slowed. Today, these rivers meander through wide valleys left between the old riverbanks. The only significant groundwater supplies found in southern Indiana are located in these sand and gravel deposits.
Two-thirds of Indiana is located north of the line of furthest glacial advance. Here we find a thick glacial till, rolling fertile prairie, and readily available groundwater. While to the south of this line, we find thin layers of discontinuous windborne soil (loess) over sandstone or limestone bedrock, steep to moderate hills, and a general lack of groundwater. We also find a unique geologic artifact, hills and hollers more like Kentucky and Missouri – the only area of the State with geological topography that predates the ice age.
The impact of the glaciers on Indiana’s available groundwater was dramatic. In northern Indiana, groundwater well capacities of 200 to over 1,000 gallons per minute can be readily found. Over most of southern Indiana, however, well capacities are less than ten gallons per minute. In fact, large capacity wells are found only along the major rivers.
For Indiana, the Pleistocene was a time of abundant, though frozen, water. Ironically, this abundance of water also left southern Indiana with its legacy of water scarcity.
David L Dahl
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