For University of Washington earth sciences professor Terry Swanson, the geologic forces that shape Earth’s surface might be measured in eons, but they’re as action-packed and exciting as any Hollywood blockbuster.

On Tuesday Swanson, a Clinton resident, delivered an animated outdoor lecture to a crowd of Coupeville residents. He explained the why, when and how of the town’s landmark Big Rock, a glacial erratic that sits on the west side of South Main Street.

Gathered in a semi-circle around Swanson under a warm, blue morning sky, people were treated to a story that stretches back thousands of years and involves such momentous characters as a sheet of ice 1,200 meters thick and a metamorphic green boulder that was pushed hundreds of miles to its final resting place.

Swanson also spent the earlier part of the morning teaching students in Coupeville schools about the unique history of Big Rock, as well as about the geologic history of the Puget Sound in general. Former Oak Harbor teacher Iris Kroon invited Swanson to Coupeville in the hopes that Swanson would write a historic marker to place in front of the erratic. Swanson agreed.

“I thought the Lion’s Club should sponsor a marker,” Kroon said. “What I want is the proper wording.”

She said she envisions the historical marker running about 100 words and encapsulating the salient geological aspects of Big Rock.

According to Swanson, a specialist in glacial geology, the rock was “one of the last things being deposited as the ice was pulling out of here,” a retreat that took place approximately 15,200 years ago. To his mind, this event is fairly recent.

“Geologically, it’s not that long ago,” he said.

Referring to a study he authored on glacial deposits throughout the Puget Sound region, Swanson said the Coupeville rock got part of its 15 minutes of fame when the piece was published last November in the academic journal Quaternary Research.

“The Coupeville erratic is famous,” Swanson said

Swanson’s quip about the celebrity of the rock brought chuckles from some members of his audience, likely inspired by the recent controversy sparked by a potential commercial relocation some town residents feel would obscure their view of the Big Rock.

Both the Town Council and, on Sept. 19, the Coupeville Planning Commission have approved plans to relocate Miriam’s Espresso from the east to the west side of South Main Street near the Big Rock.

A lawsuit recently filed by the “Oh Oh” organization, a coalition of citizens seeking to preserve the historic and scenic appeal of Coupeville, challenges the council’s decision. The group claims the council violated planning conditions set out in the town’s comprehensive plan.

Swanson mentioned the controversy both before and after his lecture, though he said he didn’t want to get embroiled in the fight.

“I’m here to talk about the rock and not the political side of this,” he said. Swanson later said he believed the town could find a middle ground on the issue. He said he feels preserving the view of Big Rock would be beneficial both to Miriam’s new business and the town residents in general.

“I’d think it’s a value to their coffee shop,” he said.

The controversy surrounding the Big Rock was a mere drop in the bucket compared to the rest of Swanson’s story, which involved the entire glacial history of Whidbey Island and its surroundings.

Swanson informed his audience that the northern Puget Sound region was covered with ice about 15,000 years ago, with one lobe of a giant glacier passing over what is now Whidbey Island before moving on to Seattle and Olympia. That lobe, he said, is responsible for all glacial deposits found in the area.

“Ice does not discriminate what it carries,” Swanson said, referring to how the glacier picked up everything from silt to giant boulders. “The erratic is here because it was situated in sediment directly deposited by the ice.”

That ice sheet, which Swanson predicted was about 3,600 feet thick, was part of what is called the Vashon Advance. When this glacier retreated 15,000 years ago, it plunked the Big Rock in Coupeville.

Though many people who look at Big Rock believe the rock is granite, Swanson said they are mistaken. The Big Rock is actually a green stone, a metamorphic rock formed from sediments put under extreme heat and pressure.

“It’s as hard as rock,” Swanson joked.

Swanson uses well-preserved erratics like Big Rock to measure past changes in global climate. Conditions around the region are particularly good for such studies.