There is a big rock sitting at the toe of Double Bluff, a rock that wasn’t there a year ago.

Roughly the size of a van, this boulder sits at the high tide line looking as though it has been on the beach since the dawn of time. It’s an impression that rings true for most people who see it for the first time, since — at over 10 tons — this rock is not the sort of thing a prankster would bring in overnight.

But anyone who thinks this has the wrong idea about how giant rocks move around. Whidbey Island locals who have walked along the bluff more than a time or two know where the rock came from; about 100 feet over their heads, where it fell out of the constantly eroding bluff face.

The other people who know about the rock’s origin and its long history are geologists. And for them, the thought of a truck-sized piece of granite falling out of the sky and plopping onto the beach is not terrifying: It’s fascinating.

Last Saturday, 40 geologists from around the nation came to Whidbey Island to see that big rock, another rock that is far larger, and some of the most visible and active geology in the United States. On a field trip from the annual Geological Society of America Conference being held in Seattle this week, the group — which included teachers, undergraduates, college professors and even a government researcher from Los Alamos — came to the island to get a cutaway view of how the Earth works, and to have a day with like-minded scientists.

“I feel like I’ve found my tribe,” said Ruth Krumhansl, a high school Earth systems teacher from New Hampshire who was just grateful to get into the field with a bunch of backpack-toting geologists.

Stopping at exposed bluff faces in Clinton and at Double Bluff, at the Coupeville Erratic (also known as the Big Rock), at a gravel pit, and at The Kettles north of Coupeville, the geologists were looking over evidence that shows just how a 60-mile-long island can be built by rivers, glaciers and rain. Terry Swanson, a geology professor at the University of Washington and a Possession Point resident, led the tour and leant his perspective on Whidbey’s several hundred thousand year history to the day.

Remarking on how easy it is to find exposed bluff faces that show distinct layers of sediment dropped by at least two glacial periods and possibly several ancient rivers, Swanson was able to give his tour group the geological equivalent of an examination of “The Visible Man.”

“In a sense, it’s like a natural laboratory,” he said.

Away from where pavement covers all traces of natural features and in a place where waves, wind and rain expose the layer cake of the ground beneath human feet, buildings and vehicles, the geologists could see how Whidbey Island grew over the course of about 200,000 years. That boulder on the beach, for example, was likely dropped by the retreat of the Cordilleran Ice Sheet during the most recent ice age, approximately 15,000 years ago.

For decades, it has been thought that this period of glaciation, in addition to one 200,000 years ago, was responsible for creating Puget Sound and its islands. But what the visiting geolgists saw last weekend shows this may not be so.

The Sound and its islands, which mark the southernmost extent of the mile-thick ice sheet, were likely a work in progress long before the glaciers covered the land. Scrambling up and sliding down the long toe of Double Bluff, the geolgists found layers of sand and rock that seemed to have been deposited by an ancient river running out of the Cascades, a layer of clay likely dropped to the bottom of an ancient lake, and woody pieces of ancient peat indicating Double Bluff was at one time a giant swamp. While the glaciers would later make a big impression on these formations (one of the giant scrape marks left behind forms the valley through which Highway 525 climbs out of Clinton) there was already a lot of land here before the ice flowed south, land that was connected with the rest of Washington.

Swanson said research he has done over the past few years could prove definitively that the island is not simply the sandy, rocky garbage left behind by retreating glaciers. To his mind, Whidbey Island’s history could read closer to this description:

About 200,000 years ago, a glacier covers the old river sediment with a heavy layer of ice, then begins to retreat. It is the sixth glacier to do so. About 125,000 years ago, rivers flowing from the Cascade mountains deposit a thick layer of sediment over the top of other layers of sediment during a time in which there is no Puget Sound. Over time, this layer would stand almost 4,000 feet atop the bedrock in the area that would become South Whidbey.

Later, as Northwest Washington became more seismically active, earthquakes eminating along Whidbey Island’s faults deform and crack the sediment layer, bending and otherwise distorting layers of rock, sand and clay. The retreat of the glacier blocks meltwaters from the mountains at some point, turning the area over Whidbey Island into a massive lake. Then, 18,000 years ago, the Cordilleran Ice Sheet rolls over the island for a seventh time, cutting valleys in the existing sediment and leaving behind its own batch of gravel and rock. While all this is going on, sea level rises by 200 feet while the level of Whidbey Island comes up 400 feet.

At present, all of this is speculation, supported by very visible evidence around the island. That evidence is good enough, Swanson said, to start bringing his undergraduate students to the island to do more research — something he’d like to start in the next year. That shouldn’t take much convincing, since UW geology students, like senior and Swanson student Ryan Muno, love to talk rock.

“It’s actually pretty cool hanging around so many geologic minds,” said Muno while in the midst of Saturday’s GSA outing.

The only thing more geologically engaging than the island’s past may be its future. Two potentially active fault lines — one on North Whidbey and one on South Whidbey — and the erosive forces of weather, time and people guarantee that Whidbey Island will be a changing place over the coming millennia. Even on the most calm of days, the island is doing what the force of gravity compels it to do; collapse into the sea. On Saturday, as UW senior Michelle Mullen sat 70 feet up Double Bluff atop an hourglass-shaped pile of sand, grit and rocks trickled down, pulled only by the unseen force. One the size of a golf ball hit her in the back.

“Ow!” she said, not bothering to turn around to see where it came from.

Left to itself, Whidbey Island would one day, after hundreds of thousands of years, erode to almost flat, then disappear under Puget Sound’s waters. But that’s unlikely to happen, as the Earth tends to keep moving, albeit too slowly for most people to notice.

Saturday’s field trip and future geological research on Whidbey Island is made possible by private land owners who allow study teams onto their property.

“We do appreciate that,” said Terry Swanson.

GSA’s conference, Geoscience Horizons, runs through today. An estimated 5,000 geolgists attended and participated in dozens of field trips around the Puget Sound area.