Mima mounds mystery solved?
Gophers moved soil uphill during wet conditions, scientist claims
OLYMPIA – A California geology professor said he has solved one of the enduring geological mysteries of the Pacific Northwest.
Emmanuel “Manny” Gabet, a geomorphologist at San Jose State University, said prehistoric generations of pocket gophers created the vast fields of Mima mounds found in south Puget Sound and in other locations around the world.
Gabet’s findings, aided by two co-researchers, were presented in December at a meeting of the American Geophysical Union in San Francisco. The conclusions have been reported by dozens of media outlets around the world, including the BBC, the Economist, Der Spiegel, Popular Science and public radio.
Not so fast, say local geologists and wildlife researchers.
“Any time a scientist says, ‘I solved the mystery of,’ almost every normal scientist starts rolling their eyes,” said University of Puget Sound geology professor Barry Goldstein. “It’s not the style most of us are accustomed to. It almost always means what you’re going to get is something pretty simplistic.
“Things are usually more complicated than that.”
Starting with the Chehalis Tribe, who thought the mounds were the result of a great flood, many have tried to explain the origin of the hillocks, each about 40 feet wide and 7 feet tall, that flow rhythmically across hundreds of acres.
The mounds are a recurring feature throughout the world but one of the largest expanses of them, and where they get their name, occurs about 10 miles south of Tumwater in the Mima Mounds Natural Area Preserve.
The idea that normal-size gophers built the mounds to get above the water table was first postulated in the 1950s, Gabet said. What makes his research novel and why he said he’s solved the mystery is the addition of a new kind of gopher to the theory: virtual gophers.
Gabet developed a computer model that contained the simulated gophers. He used data from a Mima mound study done in San Diego where iron tracer pellets showed gophers moved soil uphill in response to wet soil conditions.
“I used the data from those tracer experiments to essentially control how my virtual gophers behaved,” Gabet said. “I started with a flat surface and over time the Mima mounds in the model just started to emerge.”
Washington state scientists are receiving Gabet’s findings with all the warmth of an ice age.
“What this guy has is a simulation. It’s not evidence. It’s within the realm of possibilities,” Goldstein said. “But there are many, many other things in the realm of possibilities.”
Goldstein studies the geology behind glaciers. Research he and others have done in the area shows the local Mima mounds were formed within just a few decades and possibly much shorter. Gabet said he’s never visited Washington’s Mima mounds. His studies concentrated on Mima mound fields near Merced, Calif.
Another local geologist, Tim Walsh, a geologist with Washington’s Department of Natural Resources, also thinks Gabet is making mountains out of gopher hills.
Goldstein and Walsh said the Puget Sound mounds were formed on a terrace that bordered advancing and ebbing glaciers during the waning days of the most recent ice age – about 15,000 years ago. Water that flowed past the ice sheets and over the terrace drained into the ancient Chehalis River.
Gabet stands by his gopher theory.
“In these situations where the soil gets waterlogged they must be responding to some soil moisture cues,” he said. “They feel their burrows getting wet or their feet getting wet. That flips some switch in their behavior and they start pushing soil uphill. And that is what was found in that tracer study that was so surprising, that in some cases they push soil uphill.”
The Mima Mounds Natural Area Preserve doesn’t have gophers living there but some live in other Mima mound areas, said Gail Olson, a wildlife research scientist with the state Department of Fish and Wildlife. Olson said gopher mounds – the 6-inch-tall variety – don’t last long as they are quickly eroded by rain, wind and other animals.
Mima mounds, by contrast, occur with an almost geometric regularity. From the air it appears as if the Earth has goose bumps.