Laser mapping may help solve the mystery of the Mima Mounds

From goofy to erudite, more than three dozen theories have attempted to explain the origins of grassy mounds that dot the prairies of Southwest Washington. The latest twist won't settle the debate, but it casts the mysterious hummocks in a different light.

Laser light, that is.

Scientists used airborne laser surveys to create topographic maps that reveal new details about the so-called Mima Mounds scattered across lowlands south of Olympia and Tacoma. The technique fires 23,000 pulses a second toward the ground, and erases signals that bounce back from vegetation and buildings. The result is an exceptionally crisp image accurate to a few inches or feet.

The new maps clearly show that all of the mounds formed near the margins of retreating glaciers, supporting an idea first proposed nearly a century ago, said Robert Logan, chief of geological mapping for the Washington Department of Natural Resources.

Logan and Timothy Walsh, who leads DNR's geologic-hazards section, tweaked the old theory to fit their new observations. They presented their work in Seattle last week at the 81st annual meeting of the Northwest Scientific Association.

Fans of the gopher theory aren't buying it — but more about that later.

Standing up to 6 feet tall and measuring 30 feet across, Washington's most famous mounds are at Mima Mounds Natural Area Preserve near the town of Littlerock. But "pimpled prairies" cover about 10,000 acres and include tracts on Fort Lewis and near Yelm, Roy and Spanaway.

The story the DNR scientists deciphered from the landscape began at the end of the last ice age, about 13,500 years ago, and features gushing meltwater, sun cups and erosion. There's even a hint the ice might have retreated at breakneck speed.

As glacial lobes melted, Logan explained, dammed-up water occasionally burst free, gouging channels. The new maps show all of the mounded tracts are adjacent to outburst channels. The surging floodwaters would have carried gravel, which underlies the mounds. The topography also shows evidence that the meltwater pooled up, perhaps dammed by chunks of ice.

Cold winds blowing off the glacier would have frozen the ponded water, Logan said.

That's where the sun cups enter the picture. As anyone who's trekked across glaciers or snowfields in warm weather knows, these scalloped depressions form naturally.

Meanwhile, water would have continued to pour from the melting glaciers, washing across the sun cups and dropping sediment. When all the ice was gone, sediment collected in the sun cups could have been left behind, forming the mounds that remain today.

Washington's mounded tracts trace the path followed by the retreating glacier, and Logan said he believes they were formed in succession. In fact, he says, glacial terraces in the maps suggest four or five episodes of mound formation, each of which could have occurred in a single season. That would mean the glaciers zipped back 20 miles in five years.

"I'm convinced they broke up rapidly, much like the ice shelves we're seeing in Antarctica," Logan said.

Others want more evidence before they accept the rapid retreat.

But the basic mound-forming elements in the DNR team's explanation are the same suggested in 1913 by the eminent geologist J. Harlan Bretz.

Bretz didn't follow up on his mound theory, perhaps because he was busy uncovering evidence of cataclysmic ice-age floods that gouged out Eastern Washington's basalt badlands.

Some early explorers assumed Native Americans built the mounds, as religious or burial sites. Others have posited prehistoric fish nests, permafrost heaving and seismic shaking as the explanation.

But most theories suffer from parochialism, said Donald Johnson. Mima-like mounds occur throughout North America, including areas such as Texas, Mexico and California where glaciers didn't reach, says the retired University of Illinois geography professor. He and his wife have been visiting all the known mound sites, and are convinced they share a common and humble architect: the pocket gopher.

Mounds invariably form in places where thin topsoil is underlain by a hard layer, such as gravel, bedrock or hardpan, Johnson said. In those conditions, the industrious rodents are forced to burrow sideways, and their digging shifts soil back toward their central nest. The result is a slow buildup of dirt.

"You fast-forward over a thousand years, and you've got a nice mound," he said. Even a small mound gives gophers an advantage by elevating them above shallow water tables and making it harder for predators to dig them up. So generation after generation would make their homes in the mounds, and the mounds would continue to grow, Johnson said.

He concedes Washington's Mima Prairie doesn't show direct evidence of gopher inhabitation. But nearly every other tract he's studied does — including several in Washington.

Johnson is writing a book on the subject but doesn't kid himself that it will be the final word.

Parks and nature preserves market the mystery to draw visitors to mounded prairies, he said. "It's locked into lore, kind of like the Loch Ness monster. If the Loch Ness monster were proved to be bogus, nobody would go to Loch Ness."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com