TACOMA — A California geology professor says he’s solved one of the enduring geological mysteries of the Pacific Northwest.
Emmanuel “Manny” Gabet, a geomorphologist at San Jose State University, says 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, Wash., in the Mima Mounds Natural Area Preserve.
One of the first modern theories was that they might be American Indian burial mounds.
In the 1840s Commodore Charles Wilkes, who was exploring the region for the Navy, brought an excavation crew to the Mima Prairie with the idea they would find skeletons or artifacts in the mounds. They came up empty-handed.
Other theories have included earthquakes, glaciers, gas venting, whirlpools, gigantic gophers and, of course, aliens.
Gabet studies landscape evolution.
“We look at things like rivers, slopes and glaciers and study the processes of erosion and how sediment and soil get moved around the landscape,” he said.
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 says 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 show 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.
Gabet’s explanation might work for Merced’s area, Goldstein said, but not others.
Another local geologist also thinks Gabet is making mountains out of gopher hills.
“He started with the assumption that all mounds that people call Mima mounds have a common origin and that is a flawed approach,” said Tim Walsh, a geologist with Washington’s Department of Natural Resources. “I don’t see how you can make the leap that all mounds formed that way.”
Walsh, who studies stratigraphy, emphasized he was speaking for himself. DNR takes no position on how the mounds were formed.
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.
One peculiar aspect of the mound formation in the Littlerock area is that older and newer terraces exist above and below the terrace that contain the mounds. But those other terraces, which are made of the same material, are mound-free today.
“If pocket gophers did do this, they would just have easily been able to do it on the higher or lower ones,” Goldstein said. “Especially the newer terrace when the climate would have been warmer.”
Gabet said a water-impermeable layer of hardpan exits under or near the surface in nearly all Mima mound sites, making water saturation a common occurrence. But Goldstein and Walsh said other differences in the locations and types of soils found in different mound sites make a common origin of gopher architects highly unlikely.
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.”
Gabet said a gopher’s lifespan is about two to three years but it takes hundreds of years to build the mounds.
“So each gopher only benefits in a minor way from their own work,” he said.
A wildlife researcher who studies pocket gophers on the Mima prairie said she’s never seen the little mammals push soil uphill.
“The computer model seems to have been built on gopher behavior that I haven’t seen,” said Gail Olson, a wildlife research scientist with the state Department of Fish and Wildlife. “I don’t know of any switch that would change their behavior like that.”
Gabet said, accounting for size, Mima mounds are the largest structures built by any mammal on Earth — with the exception of humans. Olson isn’t ready to give gophers that much credit.
“They are not building anything,” she said. “They are just getting rid of dirt in their tunnels.”
Olson said she’s never seen one build a mound taller than 6 or 8 inches.
“There’s no concept of uphill or downhill,” she said. “They live in closed tunnel systems underground. I hardly have ever seen one above ground.”
If they spent time outside their burrows, Olsen said, “They would get picked off by a raptor pretty quickly.”
Olson concentrates her research on prairie wildlife, mostly focusing on pocket gophers for the past six years. The little critters are a hot topic. They are listed by the state as threatened and have been a candidate for listing under the federal Endangered Species Act for decades.
A decision is expected in March, Olson said.
The prairies of Western Washington in general don’t tend to become saturated to the point it would affect gophers, Olson said.
“(The soils) are generally well drained,” she said. “They don’t pool water. It would be a rare event to have gophers saturated with water.”
Goldstein and Walsh concur that the soils in and around Western Washington’s mounds drain quickly.
The Mima Mounds Natural Area Preserve doesn’t have gophers living there but some live in other Mima mound areas, Olson said. They occur naturally in the Scatter Creek Wildlife Area and were introduced in the West Rocky Prairie Wildlife Area. Vast mound fields also exist on Joint Base Lewis-McChord.
Olson said gopher mounds — the 6-inch-tall variety — don’t last long as they are quickly eroded by rain, wind and other animals.
“They just disappear over time,” she said. “They are fairly random, at least to us, where they occur.”
Mima mounds, by contrast, occur with an almost geometric regularity. From the air it appears as if the Earth has goosebumps.
Gabet said his model takes erosion into account.
“In the model, if I turn off the gophers, the mounds just disappear over time,” he said.
There are numerous ways to make ordinary mounds in nature, Walsh said, from permafrost to earthquake liquefaction.
Goldstein isn’t willing to cite any theory or even hypothesis about the Mima mounds’ formation but he did mention an “idea”: a major earthquake.
“If an earthquake had hit while the surface was still wet (during the ice age), the water-saturated material can form into those shapes,” he said.
Goldstein said he doesn’t lie awake at night pondering the origin of the mounds but, “It’s a feature that requires explanation. If we could really understand their formation, it may give us insight into something else as well,” he said.
Will the mystery of the mounds ever be solved?
Washington state geologists aren’t placing any bets.
“They’re kind of fun to dink around with,” Walsh said. “But nobody gets funded to study Mima mounds.”