The Seattle Fault Zone -- A Field Trip to Alki Point

Sunday, September 26th marked the first reunion of the M9.2s and a field trip to West Settle in search of evidence of activity along the Seattle Fault.  Scott took the ferry over from Bainbridge and met Kathy, Teresa, Carlie and me for an afternoon on the beach.  We drove over to Duwamish head, a prominence that juts northward from West Seattle into the mouth of Elliot Bay.  From this locale, you get a beautiful views in every direction, including the city skyline, north Puget Sound, the Kitsap Peninsula, Bainbridge Island, and Alki Beach. There is a thin strip of Alki Beach that is an old beach terrace exposed by the last Seattle fault quake in 980 A.D.  You get a good view of the area under which the fault complex stretches.  You also get a good perspective of how close the a heavily populated urban centers are to this geologic feature.

Next, we headed south of Alki Point to a portion of the roadway that shows a 2-meter change in elevation believed to be associated with the fault zone.

We then proceeded north again to an area across from Mee Kwa Mooks Park.  After hopping down off the sea wall onto the beach (it was low tide), Scott quickly found what we were looking for.  An exposure of the Olympia Beds, radiocarbon dated to 27,810+/- 130 years before present.  The Olympia Beds represent an episode that predates the most recent (Vashon) glaciation, and shows evidence of an environment that was interglacial but still cooler than today.  The geology indicates the presence of peat bogs, river beds and floodplains, and lakes.  While the Olympia Beds can be seen in other locales around Seattle, the relevance to the Seattle Fault are the folds apparent in this one location.  Precisely how the folds correlate to the fault are an area of continued research.  For us, the coolest aspect of the formation was the woody debris embedded in the muds.  It was quite the experience to hold 28,000 year-old wood.

 The group plies apart the layers of the Olympia Beds.

A finger pulls at a piece of woody bog material within the muds of the Olympia Beds

(From left) Kathy, Teresa, and Carlie examine a slice of pre-Vashon glaciation bog mud. (Despite repeated reminders that this was a geology trip, Carlie insisted on interacting with the biologicals) 

Our next stop was a beach outcrop of the Blakely formation just south of Alki Point.  The Blakely formation is essentially a gray sandstone that interbeds with some siltstone and can contain volcanic pieces of rock.  More importantly, it's about 40 million years old, so we added three zeros in age relative to the geology of our last stop.  This is an old beach environment, but the fascinating piece is that just north of the fault zone, it lies several kilometers down.  The faulting and continued earthquake activity has forced this much older rock both upward and northward, and offers some of the most compelling evidence for the Seattle fault.

The Blakely Formation sandstones on the beach south of Alki Point

Our final stop was for for victuals and pints, so we settled on Cactus where we could see Puget Sound, the Seahawks (who beat the Chargers), and engage in conversation on geology teaching and the politics of public education.  We missed Adrienne, Cathy, Catherine, and Stephanie, all who intended to come but had unforeseen issues that prevented them from attending.  Fear not, the Blog never ends, and there was already talk of a spring meeting to share experiences and plans.

A well-fed and happy bunch of middle-school earth science teachers after Cactus and a surprisingly rain-free and pleasant afternoon on the beach.