More than 30 explanation have been proposed for the origin of the Mima Mounds, but none has been proven.
Were the Mounds Created by Glacial Ice?
In 1913 glacial geologist J Harlan Bretz completed the first detailed study of the Mima Mounds. Bretz suggested that gravel, stones and soil washed onto the melting ice age glacier might have collected in pits, called suncups. As the ice melted, the sediment pits would have settled in mounded shapes.
Created by Earthquakes?
Andrew Berg suggested that mounds might form when earthquakes generate vibrational shock waves that travel through the soil. In this hypothesis, the soil would move the most where wave peaks intersect with each other (blue dots), and tend to collect as mounds in the places where the waves cancel each other out (white dots).
Origin of Mound Sediments?
Some scientists have proposed that the Mima Mounds are made of deposits from sediment rich floods from a glacially dammed lake. Other scientists added a new twist by suggesting that the mounds are sediment that collected where the water flowed around vegetation. Their views are known as fluvial deposition hypotheses.
Caused by Erosion?
Explorers and scientists since the mid-1800s have proposed a variety of mound hypotheses based on erosion from glacial meltwater. In 1988 glaciologist A.L. Washburn reviewed the evidence for and against all the mound hypotheses to date. He favored hypotheses based on runoff erosion with vegetation anchoring. In these hypotheses, runoff from glacial meltwater eroded the soil between trees or shrubs, leaving mounds around the plants.
The Work of Small Pocket Gophers?
In the 1940s Walter Dalquest and Victor Scheffer proposed that the mounds were formed by pocket gophers excavating nest chambers. Several scientists continue to work on this hypothesis today. They suggest that after the glaciers retreated and vegetation grew on the thin soil, pocket gophers came in from unglaciated country. The gophers began to dig nest chambers, but encountered the dense layer of glacial debris beneath the soil. They moved soil and pebbles upward, forming a mound. Pocket gophers are territorial; the center of each mound is the center of a pocket gopher's territory.
The Result of Permafrost Cracking?
This explanation suggests that frozen ground—or permafrost—at the end of the glacier cracked into many-sided shapes. Ice formed wedges in the cracks. As the climate warmed and the ice melted, the soil was left in mounded shapes.