The Earth started to cool about 50 million years ago. Then about 2.6 million years ago Earth entered another "ice age" - a cold period when ice sheets advanced and retreated in North America and other parts of the world. The graph shows the most recent 4 advances - called "glaciations" or "glacial periods" (labelled on graph). The last glacial period, popularly known as "the Ice Age", was the most recent glacial period within the Pleistocene epoch. Scientists consider this "ice age" to be merely the latest glacial event in a much larger ice age, one that dates back 2.6 million years and has seen multiple glacial events. The formation of the Glacial Lake Great Falls and the Sonkin Sag happened in the near the end of the most recent glacial period (~13,000 years ago), and may have happened during previous glaciations as well.
Confusing! . . .