Kitsap Geology: The Blakeley Formation

The Kitsap Peninsula’s geologic history span tens of millions of years. There’s the Crescent Formation: the vast mass of 50 million-year-old basalt rock that forms the core of the Blue Hills southwest of Bremerton. And, although you may not know it, most of us are also familiar with the most recent geologic story, which makes up most of the peninsula’s bulk. Our homes and towns on the peninsula are built on top of the enormous pile of debris left behind by the retreating ice age glaciers which filled the Puget Lowlands up to 17,000 years ago, this is collectively known as the Vashon Stade.

Blakeley Formation exposed at Manchester State Park

But there is a third geologic story. Named for its prominence around Blakely Harbor on Bainbridge Island, the Blakely formation is a 36-34 million-year-old stack of sedimentary rocks, transported from the east. It’s accumulation occurred between the emplacement of the Crescent Formation and the deposition of the Vashon Shade.





Today, on the Kitsap Peninsula, the sight of the distant Cascade Mountains to the east accentuated by the hulking mass of Mount Rainier is a mundane one. But this view wasn’t always here. The Cascade Range is relatively young, first beginning to rise around 40 million years ago. Mount Rainier and our other fire mountains of the northwest haven’t been here for nearly that long. Average stratovolcanoes only erupt for about a million years. Thus over the past 40 million years, there have been entire generations of cascade volcanoes that have erupted and then been eroded away. The Blakely Formation is one piece of evidence of their existence.

Many people living in the Puget Lowland have heard of the volcanic phenomena known as lahars or mudflows. Mount Rainier and the other volcanoes which dot the skyline are mantled with a blanket of permanent ice and snow. When these volcanoes erupt, the molten material falling upon the glaciers and snowfields melt them and create a slurry of debris that roars down valley and buries everything in its path. The deposits left behind by recent mudflows are what underlies many of the towns and cities at the foot of Mount Rainier. But just as volcanoes have been erupting in the cascades for 40 million years, lahars have been flooding the lowlands just as long.

A Lahar raging down a river valley from Mount Ruapehu in New Zealand. Photo Credit goes to Geoff Mackley (Who has some incredible Volcanic footage and photography if you look him up)

30-40 million years ago, the ancient volcanoes of the Cascades sent lahars and volcanic floods careening straight into the ocean. Puget Sound didn’t exist back then, and the Olympic Mountains, which are such a centerpiece of our skyline to the west, didn’t rise above sea level until 10-20 million years ago, long after our story takes place. As these floods settled on the ocean floor, they formed an extremely thick stack of sandstones, siltstones, and conglomerates just off the coast, building up to several hundred meters thick under the waves. After solidifying, these rocks were folded and shoved upward, probably around the same time that the Olympics and Kitsap’s Blue Hills rocks were uplifted. Finally, after being re-uplifted, the successive invasions of ice into the Puget Lowland buried the Blakely Formation in a thick layer of glacial silt and gravels. Since then, erosion has exposed the formation in several places.

Today there are two main sites where you can view the Blakely Formation on the Kitsap Peninsula proper. The first is Manchester State Park; this state park is located east of Port Orchard and requires a Discovery Pass to visit, which are 11 dollars for a day pass or 35 dollars for an annual pass. Excellent exposures of the Blakely formation can be seen on the shore of the park, with good exposures of more recent glacial deposits in places as well.

The  Blakely Formation as exposed on the Shores of Rich Passage at Manchester State Park. Photo by Micah K. 

The other place in Kitsap where you can view the Blakely Formation is probably much closer to home and convenient for most people but is a little trickier to access. Driving south down Tracyton Beach road, park at the first pullout you encounter on the south at the coast (or the last on the left if driving north). From the pullout, scramble down a small (and steep!) embankment, and you will be standing directly on top of Blakely Formation rocks. Proceeding south along the shore, you will find different layers, and some amazing textbook examples of a rock formation called tafoni, which is also known as honeycomb weathering. Be sure to step gingerly and be careful around these sensitive formations so that others may enjoy them in the future! Just below the roots of a large pine tree is a layer that I have interpreted to be from a volcanic lahar, with large rocks perpetually frozen in the silty matrix that surrounds it. It is probably best to observe this outcrop at low tide, as high tide forces you to scramble along the steep rocks.

The Blakely Formation as exposed on the shore of the Port Washington Narrows along Tracyton Beach Road. Photo by Micah K.