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. 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.
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.