A geologist who built a doghouse with some dusty plywood believes he may have discovered the obvious to explain the mysterious prairie mounds that dot the American landscape.
The mounds, also known as Mima or pimple mounds, are scattered across parts of Washington, Oregon, California, Wyoming, Colorado, northern New Mexico, southeastern Oklahoma, eastern Texas, Arkansas and western Louisiana. They long have baffled scientists.
Andrew Berg, a geologist for the U.S. Bureau of Mines, believes the mounds were formed by earthquakes thousands of years ago. Some other scientists are intrigued by his theory but say more research is needed.
“Right after the Mt. St. Helens ashfall 10 years ago, I was working in my back yard here in Spokane on a kennel for my dog. There was a plywood board on the ground. It had a little dusting of St. Helens volcanic ash on it,” Berg said.
He laid some 2-by-4s on the board to nail a frame.
“I started hammering away and, all of the sudden, I looked at the board and here were these little bumps of ash.”
It’s the same principle that clumps sawdust when it is vibrated by a table saw.
Berg, 64, wondered if the vibrations on a larger scale, such as seismic activity, might have created the hillocks.
So he tried again, this time with a larger sheet of plywood and some loess, a fine-grained soil. “I tapped it with a hammer and sure enough, vibrations caused little miniature mounds,” Berg said.
The hammer creates shock waves. Some parts of the board vibrate hard and others remain still as the waves intersect. Similar wave interference during earthquakes could have created the Mima mounds in areas where loose soil sits on a hard surface, such as rock, Berg said.
The tiny mounds on his board were not wiped out by subsequent blows of the hammer, leading Berg to believe the mounds are stable.
He said his theory could help explain why the mounds, first written about 150 years ago, form in such different climates.
At Mima Prairie, south of Olympia, Wash., there are hundreds of mounds ranging from 8 feet to 50 feet across and up to 6 feet high. They’re not always easy to see except from the air.
Some scientists have said they were built by gophers or formed by glacial action. No one really knows.
“There’s probably almost as many ideas on the origin of these things as there are people who have looked at them,” said Charles Higgins, a geology professor at UC Davis.
Higgins said he doesn’t know if Berg is correct, but he admits that the theory answers some geological questions. “I’m inclined to think that the mounds are more the result of some kind of field stress--desiccation cracking or frost cracking,” he said.
“I would like to have seismologists either verify it (the theory) or set it aside. I hope it’s widely discussed and kicked around,” Berg said.
Albert Lincoln Washburn, a retired geology professor at the University of Washington, said the idea needs research.
“It’s intriguing, but the basic evidence remains to be forthcoming,” Washburn said. “I don’t think it’s been proved that all these mounds in different parts of the country have the same origin, by any means.”