How did that get here?

EDMONDS — After untold millennia of glacier surfing, this big rock is taking a quick, 12,000-year break.

Geologists believe a boulder located in a mobile-home park just off Edmonds Way is a

n “erratic,” a stone deposited in the area by glaciers during the ice age.

Looking at the rock — roughly 12 feet high by 10 feet wide by 15 feet long — “erratic” would seem to be a misnomer.

It seems very stable.

It’s about the size of a camper, estimates Dave Tucker, a research associate in geology at Western Washington University in Bellingham. Tucker discusses the boulder, and others, on his website, titled Northwest Geology Field Trips.

Many erratics are located around the Puget Sound area, from some as big as a house to beach-ball-sized stones buried in our back yards, said Donn Charnley, a professor emeritus of geology at Shoreline Community College.

The stones were picked up by glaciers and moved, maybe many times, during the ice age that began about 2.5 million years ago, geologists say. In this area, the ice sheets receded for good about 12,000 years ago, Charnley said. The stones were left behind.

In some cases, the large stones remain in neighborhoods or small towns, and houses or streets are built around them, said Dale Middleton of Seattle.

He is vice president of the “Puget Lobe” chapter of the Ice Age Floods Institute, a study group of geology enthusiasts and others in Washington, Oregon, Idaho and Montana. The local chapter takes its name from the Puget lobe, the river of ice that carved out Puget Sound. The group meets in Edmonds and draws people from around the region.

Other examples of large erratics include the Wedgwood Rock in the Ravenna-Bryant neighborhood of northeast Seattle, and Big Rock, in Coupeville on Whidbey Island.

Another large stone on Whidbey, the Waterman erratic, is located in the woods near Langley. This “hulking brute,” as Tucker calls it, is 33 feet tall, he says. The one in Coupeville is about 22 feet high.

Sometimes, developers don’t leave the big rocks intact.

“They’ll just blast those suckers out of there,” Middleton said.

An erratic is any stone that’s been transported from its place of origin, Charnley said. It’s identifiable by having a different composition than the stones and the ground around it.

Geologists believe the Edmonds erratic, like many others in this area, is from the Fraser River valley in southern British Columbia, having similar characteristics to stones in that area.

The stone is a conglomerate, meaning it’s made up of many stones cemented together. Several types of substances, such as quartz, iron oxide, calcite or clay, can serve as the cement, Charnley said.

The weight of the ground above the stone and the moving plates of the surface of the Earth presses, squeezes and heats the minerals so they meld together.

Charnley believes the Edmonds boulder is held together by quartz.

“It’s been really cooked. It’s really hard,” said Charnley, 83, of Shoreline. If someone were to take a hammer to it, “it would probably bounce right off of it.”

Tucker describes the Edmonds erratic as a breccia, a type of conglomerate in which the smaller stones that make it up are angular, rather than rounded off by movement over time. Breccias are often found close to their places of origin, which sometimes are volcanic.

It’s impossible to tell exactly where the rock came from unless a cross section of the stone can be examined, Charnley said. Because of the boulder’s hardness, however, it could take a jackhammer to get a large piece off it, he said.

Still, at some point, someone managed to drill into the top enough to install a metal post of some type, to which is attached a rope.

Neighborhood kids come over and climb on the boulder, though either the rope or its attachment to the post is now broken, according to Zina Stewart, who lives in the mobile home next to the rock. Someone also has set up a small, improvised bench on another part of the top of the rock.

The landscaped area around the boulder is well kept, with the rock’s smooth, undulating surface almost fully exposed.

Considering its history, not much is likely to faze this massive mass of mineral. Its thousands of years in Edmonds have been but a tick on the clock of its life.

Charnley believes the stone is anywhere between 50 million and 300 million years old.

“It’s been around awhile,” he said.

Bill Sheets: 425-339-3439; sheets@heraldnet.com.

More boulders

For more information on ice age boulders in the Puget Sound area, go to Northwest Geology Field Trips at http://nwgeology.wordpress.com. For information on the Ice Age Floods Institute, go to www.iafi.org.