Monday, October 17, 2011
Despite its significance in shaping the modern Puget Sound landscape, the Vashon glaciation was remarkably brief (geologically) with the ice advancing and retreating across the southern Lowland in a thousand years, perhaps much less. Saturday's field trip looked at a fascinating legacy of the rapid recession.
As the ice melted northwards towards Tacoma, it may have trapped meltwater in a large lake beneath the ice in what is now the Puyallup Valley. When the pressure got high enough, the water burst out through spillways on the western edge of the valley and poured across Lakewood and Steilacoom, eroding and redepositing enormous volumes of gravel in a complex pattern of broad channels and terraces. LIDAR images capture a remarkable fluvial landscape stretching across the Lakewood and Steilacoom area, one not fully appreciated when stuck in Fort Lewis traffic on I-5.
The outburst floods (Jokulhaups), of which there were many, reached Lake Russell (the glacial lake that occupied South Puget Sound at that time) at Dupont and later near the mouth of Chambers Creek, where they formed large, steep, coarse-grained deltas. The tops of the deltas are 160-200 feet above the modern shoreline, corresponding to the level of Lake Russell.
Much of 20th-Century Seattle and Tacoma were built from sand and gravel mined from the Chambers Creek delta, but that pit ran out of room to grow and has been replaced by a park and a golf course (Pioneer: October 2010). Since then, the region has built its foundations and its roads and its bridges with aggregate from this paleo-delta in Dupont (aerial image), which lies just north of Sequalitchew Creek.
Present three dozen geologists with a 160' cliff of gravel foreset beds and they all get pretty excited. The afternoon sun was warming the gravel face and we were entertained by a near constant rain of pebbles. It was probably a good thing we weren't allowed to actually inspect the face carefully - we might have lost someone under a large pile of clean gravel. On the other hand, it would have been interesting to look more closely for clues as to how quickly this delta originally formed.
It was great that the folks from CalPortland were willing to show us around. Credit for the day goes to Kathy and Darrell and Matthew, who've put together a great story and led a great trip.