Geology

The bluffs of Discovery Park on the South Beach are a perfect example of the layers left behind by the Vashon Ice sheet that covered the Puget Sound in the last ice age around 15,000 years ago. When you look at these bluffs you can clearly see the lines that define each of the layers. The top layer in the picture below was rocks and other materials that were dragged underneath the glacier as it was moving forward during the ice age and were deposited as the glacier receded. The second layer was medium sized bits of sand and rock that were deposited by glacial meltwater and mountain streams that were faster moving compared to the lakes created by glacial meltwaters. Because the water was faster moving only the larger pieces of sand could be deposited, the silts were too small to be deposited until they reached slower moving water.
The layers ordered in this way because the silts are deposited at the bottom of the lakes created by the glacier blocking a river or faster moving body of water that builds up in an area. Then when the glacier moves forward it pushes the sands that were deposited in the faster moving water on top of the silt/clay layer. The till is still underneath the glacier and is moved on top of the sand layer and when the glacier recedes the till is deposited on the top layer. The Lawton clay is a layer that can be very dangerous in this area. Clay is impermeable to water and causes a build up of water on top of the layer, which can cause the layers of sand and rocks on top of it to slide off. Landslides are very common in this area during the wet winter because of this.
1. Vashon till
Mixed unsorted, boulders, rocks, sand, silt, and clay, deposited by the glacier.
2. Esperance Sand
Loose, yellow-tinted sand deposited by glacial meltwater and mountain streams.
3. Lawton Clay
Dark blue-grey clay and silt which accumulated at the base of a huge freshwater lake created by glacial meltwaters.
4. Kitsap Formation
Sediments that accumulated in lakes and streams before the last glacier pushed over the Puget Sound lowlands


Picture from Geologic Hazards in Seattle
By Donald W. Tubbs and Thomas Dunne
http://www.tubbs.com/geohaz77/geohaz77.htm

For more information on the geology of Discovery Park see: 

Puget Sound Under Ice on the Department of Ecology's Puget Sound Shorelines 

http://www.ecy.wa.gov/programs/sea/pugetsound/tour/geology.html