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

On Tuesday, Swanson — a resident of Clinton — delivered an animated outdoor lecture to a crowd of Coupeville residents who desired to learn the why, when and how of the town’s 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, folks 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 about the unique history of Big Rock, as well as about the geologic history of Puget Sound in general. It was former Oak Harbor teacher Iris Kroon who 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 calendar years ago. “Geologically, it’s not that long ago,” he added.

“The Coupeville erratic is famous,” Swanson said, referring to his study about glacial deposits throughout the Puget Sound region that was published last November in the academic journal Quaternary Research.

Swanson’s quip about the celebrity of the rock brought chuckles from some members of the audience, likely inspired by the recent controversy sparked by a potential commercial relocation that some town residents feel would obscure the 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 S. Main St. 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 — challenged the council’s decision on the basis they violated planning conditions set out in the 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, though he added that “there’s always a way of working things out if we don’t polarize each side.”

Swanson later said he believed the town could find a middle ground on the issue, and that he felt preserving a view of Big Rock would be beneficial both to Miriam’s new business and the town folk in general. “I’d think it’s a value to their coffee shop,” he said.

The controversy currently 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 the crowd that the northern Puget Sound region was glaciated about 15,000 years ago, with one lobe (the Puget lobe) diverging to pass through what is now Whidbey Island before moving on to Seattle and Olympia. The Puget 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,” he added.

That ice sheet, which Swanson predicted was about 1,200 meters (approx. 3,600 feet) thick, was part of what is called the Vashon Advance. When it retreated over 15,000 years ago, it plunked Big Rock in Coupeville — no doubt unaware of the future controversy it was causing.

Many folks believe the rock is granite, but Swanson debunked this idea. Big Rock is actually a Green Stone, a metamorphic rock, and “it’s as hard as rock,” Swanson joked.

“It’s very resistant to weathering,” he said. Swanson added that he uses such well-preserved erratics to measure past changes in global climate. Conditions around the region are particularly good for such studies.

“I use the glacial record as a proxy to understand climate change,” Swanson explained.