Visitors to North Georgia from Minnesota or Wisconsin are sometimes surprised by Lake Lanier. Frequent changes in water level aren’t the norm in their own home states. The difference is that in Georgia, all lakes are artificial. Some 25,000 years ago, during the latest of four known “ice ages,” giant glaciers covered half of North America. The ice never made it any farther south than Ohio and Indiana. The massive Laurentide ice sheet, up to 10,000 feet thick, put enormous pressure on the bedrock underneath.