Ferries are an integral part of today’s Salish Sea region. But thousands of years ago, you could have simply walked across the Salish Sea, that is, if you were a skilled ice climber!

Back then, this area lay crushed below a creeping sheet of ice. Formed in Canada about 25,000 years in the past, part of it had slowly bulldozed its way south over the border as a powerful ice blanket thousands of feet thick known today as the Puget Lobe. As it crept steadily southward, the Olympic and Cascade mountain ranges on either side were barely visible above the marauding ice. The gravel, rocks, and boulders carried at the glacier’s base, along with gushing melt-water, relentlessly ground away at the terrain below.

When it finally moved over what is now downtown Seattle, the glacier grew in thickness until it was as tall as three Columbia Centers stacked on top of one another. (That’s over 3,000 feet!) As its overall shape, height, and weight increased over time, the terrain beneath was continuously carved, sculpted, and molded. Moreover, the ice had choked off the routes that water had taken from our area out to the Pacific Ocean. In other words, the Strait of Juan de Fuca was blocked, and melt-water had to flow south and down to the ocean through the Chehalis River valley.

Eventually, as our planet warmed, the Big Thaw began, and the moving ice at last came to a halt about a dozen miles south of what is now Olympia. As the frozen barrier melted and retreated over the next thousand years, the mighty Pacific Ocean could at last make its entrance. It slowly rolled in through an ice-and-boulder-carved trench above the Olympic Peninsula, to become the Strait of Juan de Fuca. Salt water also crept in to fill another gouge running northward into British Columbia: the Strait of Georgia.

The ice sheet advanced and retreated many times, each time digging out older glacial material and depositing a new layers. When the last ice sheet melted back to the north, it left behind a host of giant boulders (“glacial erratics”). Many erratics can be seen today in their final resting places.

Lands to the east of the Salish Sea also showcase our glacier’s travels. A drive into the Cascade Range reveals the effects of moving ice; as the glacier crept south, it dammed up the lower ends of several river valleys that ultimately filled to become today’s mountain lakes. A particularly scenic stretch lies between Seattle and the town of North Bend, where ancient ice carved the terrain in dramatic fashion.

Thus the forces of a historic glacier influenced northwest topography: where we build, where rivers empty into the Salish Sea, where wetlands lie, where harbors offer safe haven, where fish return upstream to spawn. Northwest residents are privileged to live in a unique water wonderland carved by ice.

And for a brief video tour or all of this, geologist Greg Geehan shows the landscape carved by glaciers 17,000 years ago. This 2 minute extract comes from just one chapter in an entrancing two hour video: The Geological Formation of Bainbridge Island. Find out about other chapters by visiting bainbridgegeology.com

Producer-Director: Cameron Snow; Lead-Camera: Cathy Bellefeuille

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Thanks so much for your interest and your support.