Scientists have proposed more than 30 explanations for the origin of the Mima Mounds, but none has been proven. In 1913 glacial geologist J Harlan Bretz completed the first detailed study of the mounds and suggested that gravel, stones, and soil washed onto a melting ice age glacier collected in pits called suncups and later settled into mounded shapes as the ice melted. Andrew Berg proposed that earthquakes might create mounds through vibrational shock waves moving through the soil, with soil collecting where the waves cancel each other out. Some scientists have argued that the mounds consist of sediment deposited by floods from a glacially dammed lake, while others suggested that sediment collected where water flowed around vegetation, ideas known as fluvial deposition hypotheses. Since the mid-1800s, explorers and scientists have also proposed erosion hypotheses tied to glacial meltwater, and in 1988 glaciologist A.L. Washburn reviewed the evidence for and against all mound hypotheses to date and favored runoff erosion with vegetation anchoring, in which meltwater eroded soil between trees or shrubs and left mounds around the plants. In the 1940s Walter Dalquest and Victor Scheffer proposed that pocket gophers formed the mounds by excavating nest chambers after glaciers retreated and vegetation grew on thin soil above dense glacial debris, moving soil and pebbles upward into mounds, with each mound centered on a pocket gopher’s territory. Another explanation holds that permafrost at the end of the glacier cracked into many-sided shapes, ice wedges formed in the cracks, and warming later left the soil in mounded forms.