 Evidence of the last glacial period, the Wisconsin glaciation from about 75,000 to 11,000 years ago, is visible as long scratches, or striations, and crescent-shaped gouges created by material carried along at the base of the ice.As the climate warmed, the glaciers melted and receded, leaving boulders that had been carried or further south from their original locations.These boulders, or glacial erratics, lie in valleys and on mountaintops, including Bubble Rock on the South Bubble.