The shores of the Great Lakes are as diverse as ocean shores, featuring high and low erosive and non-erosive cliffs and bluffs, low plains, sandy beaches, dunes, barriers and wetlands. Bedrock sea cliffs are common along the coasts of the Great Lakes, such as the shale cliffs found along the south shore of Lake Erie.
Geologically, the Great Lakes are relatively young, having been formed by glacial action during the Pleistocene period. Prior to the glacial age, the area occupied by Lake Superior was a broad valley and the area occupied by the other lakes was a spreading plain. During the ice period, glaciers deepened the bed of Lake Superior and gouged deep depressions, forming the beds of the other lakes.
As the ice sheet retreated, fingers of ice remained in the depression, rimmed by glacial moraines and outwash plains. Lakes were formed when the ice melted. Successive advance and retreats of the ice caps changed the drainage of the lake region until about 10,000 years age. Then, the northern part of the area began to rise, or rebound, as the weight of the ice sheets disappeared, causing the lakes to drain into the St. Lawrence through what is now called the Niagara River.