Qualla: Glacial Erratic Boulders of the Puget Sound Region

Glacial Erratic Boulders of the Puget Sound Region: Introduction

Wed, 16 Sep 2026 00:00:00 GMT

Photo credit Brianhe, CC BY-SA 4.0. On a quiet residential street in northeast Seattle there is a rock roughly nineteen feet tall and weighing something on the order of seven hundred tons. Houses stand on either side of it. It is gneiss, and its mineral signature matches Mount Erie on Fidalgo Island, about 89 kilometres to the north - which is to say that this boulder did not roll there, did not wash there, and was not put there by anyone. It was carried, frozen into the base of an ice sheet, and set down roughly 14,000 years ago when the ice let go. Then a neighbourhood grew up around it.

Image: Brianhe | CC BY-SA 4.0

Glacial Erratic Boulders of the Puget Sound Region: Canadian Rocks in the American Lowland

Wed, 16 Sep 2026 00:00:00 GMT

Photo credit Dennis Bratland, CC BY-SA 4.0. During the Pleistocene, a tongue of the Cordilleran ice sheet called the Puget Lobe pushed south out of British Columbia and down this basin, reaching its maximum extent near present-day Yelm. It carved Puget Sound itself along with most of the region's recognizable landforms, an...

Image: Dennis Bratland | CC BY-SA 4.0

Glacial Erratic Boulders of the Puget Sound Region: A City Built on Debris

Wed, 16 Sep 2026 00:00:00 GMT

Photo credit Dennis Bratland, CC BY-SA 4.0. Seattle sits on the evidence. Roughly eighty per cent of the city's soil is glacial drift, most of it Vashon till laid down during the last major glaciation, and nearly every one of its famous hills is a landform the ice made. Beacon Hill, First Hill, Capitol Hill, and Queen Anne...

Image: Dennis Bratland | CC BY-SA 4.0

Glacial Erratic Boulders of the Puget Sound Region: The Rock in Wedgwood

Wed, 16 Sep 2026 00:00:00 GMT

Photo credit Dennis Bratland, CC BY-SA 4.0. The Seattle boulder has a neighbourhood named around it - Wedgwood Rock, in the Wedgwood district, one of a small handful of truly large erratics in the Puget Sound area. Nineteen feet at its tallest and hundreds of tons in mass, it is the kind of object that seems to violate the...

Image: Dennis Bratland | CC BY-SA 4.0

Glacial Erratic Boulders of the Puget Sound Region: The One Hiding in the Trees

Wed, 16 Sep 2026 00:00:00 GMT

Photo credit Dennis Bratland, CC BY-SA 4.0. The largest erratics are not all in cities. On Whidbey Island, in the second-growth forest of Saratoga Woods Park, the Waterman erratic stands about thirty-eight feet high with a circumference of roughly 135 feet - a greenschist monolith with chlorite and epidote in its mineralog...

Image: Dennis Bratland | CC BY-SA 4.0

Glacial Erratic Boulders of the Puget Sound Region: Why Geologists Chase Them

Wed, 16 Sep 2026 00:00:00 GMT

Photo credit Taylordw, CC BY-SA 4.0. Erratics are more than curiosities. Match a boulder's rock type to its source outcrop and you have a data point on where the ice came from and how it flowed; plot enough of them and the flow lines of a vanished ice sheet start to appear on the map. That is the premise behind the ...

Image: Taylordw | CC BY-SA 4.0