Abstract
Low, intersecting ridges, 3-10 feet high and 75-500 feet wide, with intervening depressions form a fracture pattern which is strikingly revealed on airplane photos of the flat Lake Agassiz plain. The ridges occupy the axial part of the old lake basin and are known to extend from Fargo, North Dakota, northward to well beyond the Canadian boundary. Because the ridges are composed entirely of the underlying lake clays and surface soils, they cannot be explained by ordinary agents, such as wind, waves and currents, or glaciers. It is proposed that the ridges represent frozen-ground structures formed during retreat of the late Wisconsin ice. This is supported by the occurrence of periglacial involutions, fossil ice wedges, and polygonal and network soil patterns in the lake sediments.
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