One of the exciting, and for many people scary, aspects of Seattle is its dynamic geology. It is a feature, or perhaps a bug, that people experience every day, from traveling the city’s glacially sculpted hills and valleys to worrying about the next Big One to seep water infiltrating foundations. Few places showcase the dynamic better than the riverbank featured in my previous newsletter.
Located near the mouth of the Duwamish River, the riparian sediments are part of həʔapus Village Park & Shoreline Habitat, formerly known as Terminal 107 park. (The Westernized spelling of the Lushootseed word is haapoos, pronounced “ha-ah-poos,” and is the name of a small stream draining across a flat on the west side of Duwamish River.) The site initially drew the attention of scientists in 1975, when workers found shells during a permit application for a barge terminal. Archaeologists concluded that the area was a midden, or refuse site, where people had lived and then tossed the remains of their food, including several types of clam, between the years 670 and 1700.
The reports didn't mention though that there were other clams in the nearby riverbank that died about 500 years before people apparently began using the site. What makes the mollusks interesting is that they had lived intertidally and were now well above the tide line. We can blame California for this situation.
Think of Seattle as trapped in a slow motion collision. To the south are two train cars: the Sierra Nevadas and Oregon’s Coast Range. Plate tectonic movement is shoving the California mountains north into the Oregon ones, which in turn ram into Washington. But the Evergreen state cannot move because Canada acts as dead end and resists any northward land migration. As a result, the Seattle area is being squeezed between its two neighbors, creating what geologists call the Seattle Fault zone. Like any good fault, it periodically relieves its stress by cracking.
The most recent break occurred 1,100 years ago when an earthquake raised those long dead clams. One minute they were resting quietly in their intertidal cemetery and the next they were thrust up 20 feet, creating the bank of sediments along the Duwamish River. They were not alone in riding the quake movement.
Topping that bank above the shells is a layer of dark gray sand, which was originally part of Mount Rainier. About 1,100 years ago, during what is known as the Fryingpan Creek episode, the mountain erupted and generated a lahar, or concrete-like slurry of rock, sediment, water, and debris, that poured out the White River/Duwamish River drainage into Puget Sound. It was one of several lahars that deposited material into Puget Sound over the past 5,600 years.
The first occurred during a stupendous eruption of Rainier, when the top 1,000 feet or so blew off. At the time, an arm of Puget Sound extended up what is now the Duwamish valley past Renton and Kent to Auburn. When Rainier lost its head, the resulting lahar cascaded at speeds in excess of 130mph down to the Sound, pushing the shoreline north. Each subsequent blast generated another lahar that grew the Duwamish valley and shrank the Sound. After one final, large scale eruption about 1,100 years ago, the lahar sediments pushed the mouth of the Duwamish to its present location. It is these sands that rest atop the invertebrate rich riverbank, all of which were subsequently uplifted by the Seattle Fault.
For the Indigenous people who lived here 1,100 years ago, these epic events had a profound effect; many local stories reference supernatural serpent (a’yahos) power sites that are associated with ground shaking. These stories helped provide a way to understand and respect the power of the natural world.
I think modern residents are beginning to understand the deep connection between the geologic world and the human world. Seattleites know that our dynamic geology is what makes this such a beautiful place to live but we also are learning that we inhabit a place with a great potential for catastrophic change. We have been lucky that no geological event as significant as those recorded in the bank of the Duwamish has happened but we would be wise to heed the evidence. If you live in a landscape of young and restless geology, then you need to be prepared for the consequences.
Want to know more about my upcoming book Homewaters: A Human and Natural History of Puget Sound? Here’s a link to an interview I did with Crosscut’s Hannah Weinberger. It really captures the spirit of the book.
Feb 18 - 7pm - I will be leading my popular Who’s Watching You virtual walk exploring terra cotta and carved animals downtown.
You can follow me on Twitter @geologywriter.
I shared this image recently on Twitter so some of you know what it is. For those who haven’t seen it before, what is this wonderfully beautiful natural curiousity? I will provide an answer in my next newsletter.