The layers of sand and clay exposed in these cliffs display a remarkable geological record of the last stages of the Great Ice Age. Unique in North America, they have attracted worldwide scientific interest. The first 46 metres (150 feet) of sediments contain fossil plants and animals that were deposited in a large river delta during the first advance of the Wisconsinan glacier some 70,000 years ago. They are covered by 61 metres (200 feet) of boulder clay and sand in alternating layers left by four subsequent advances and retreats of ice. The final withdrawal of the glacier occurred some 12,000 years ago.