Charles Wilkes, an American explorer and naval officer, discovered in 1842 a huge field
of mounds on the Mima Prairie in western Washington (USA). They were many feet in
diameter, standing as high as a man, and surrounded by cobblestones the size of
footballs. Digging into the mounds, he discovered fertile, black prairie soil with
walnut-sized pebbles. Wilkes resolved that the mounds had been made by local Native
Americans, and that it must have “required the united efforts of a whole tribe.”
Louis Agassiz, a 19th century geologist and zoologist, believed the mounds to be fish
nests, left over from ancient times. Other scientists have suggested that they are the
remains of uprooted trees or gopher mounds. Others think they are a product of the last
ice age (about 12,000 years ago). According to this ice age theory, cracks were created
between frozen chunks of soil and water from melting glaciers made the spaces wider,
washing away all but the heaviest stones. Later, the mounds were created among the rocks
when the blocks of ice melted and released the gravel and soil they carried.
This may be the most reasonable explanation, but similar mounds also appear around the
world, especially in the western United States, Argentina, and South Africa. Many of
these “pimple mounds” occur in regions that could not have been affected by glaciation.