The cliffs at Discovery Park in Seattle offer a glimpse into the past, revealing layers of sediment left behind by advancing and retreating glaciers. This “layer cake” of rock tells the story of the Cordilleran Ice Sheet’s movement over the Seattle area during the most recent ice age.
Before we dive into the specific layers, let’s rewind time. Over 100,000 years ago, Seattle’s climate was similar to today, with a river system flowing north. As the Earth’s climate cooled and became wetter, the Cordilleran Ice Sheet began to form in what is now southeast Alaska and British Columbia.
Fast forward to around 19,000 years ago. The massive ice sheet reached the Canada-US border, pushing southward and splitting into two lobes. One lobe went southwest down the Strait of Juan de Fuca, while the other, the Puget Lobe, advanced south over the Puget Sound region. When this lobe reached Port Townsend, it blocked the existing river, forming a giant proglacial lake.
By 18,000 years ago, the unstoppable ice sheet had overridden the lake and covered Seattle. Water was forced to find a new route south through the Chehalis River system. Around 16,900 years ago, the glacier reached its maximum extent, pushing all the way to Olympia and reaching thicknesses of up to 3,000 feet over Seattle.
Now, let’s explore the layers of sediment visible at Discovery Park:
- Olympia Formation: This is the oldest layer, formed before the most recent glacial advance. It consists of sand, clay, and silt deposited by a river system in a non-glacial environment. Imagine a climate similar to Seattle’s present-day with streams, ponds, and backwaters.
- Lawton Formation: As the ice sheet approached Seattle, a lake formed at the edge of the glacier. This layer is made up of dark clay deposited on the bottom of that lake. The fine-grained materials suggest deep, calm water.
- Esperance Formation: As the ice got even closer, the particles deposited changed. This layer consists of sand, with some gravel lenses, deposited by glacial meltwater in a high-energy environment.
- Vashon Formation: This layer, not visible at this specific location but found nearby, is the glacial till left behind by the retreating ice sheet. It’s a mix of all sorts of materials – clay, silt, sand, pebbles, and boulders – deposited as the glacier melted.
These layers at Discovery Park serve as a record of Seattle’s glacial past, offering a window into a time when massive ice sheets ruled the landscape.
Click here to read a more detailed article written by Dale Lehman, President of the Puget Lobe Chapter, about this interesting glacial feature.