Geologic and Recent History

The Geologic History of the Puget Sound region has been defined by two major processes: the subduction of the Juan de Fuca Plate, and the Puget Lobe of the Cordilleran Ice Sheet during the last ice age. Subduction is responsible for the volcanism we see in the Cascades as well as the accreted terrain we know as the Olympic mountains. The Puget Lobe was able to advance down the Puget Trough roughly 18,000 years ago, while alpine glacier grew simultaneously. As the ice sheet advanced a glacial lake formed at its base because it was the lowest terrain that all the river drained into. This created a layer of silt that was eventually covered by a layer of sand as the ice sheet retreated. The Cordilleran Ice Sheet retreated rapidly around 16,000 years ago, and as terrain was relieved of the weight of the ice, it isostatically rebounded. Nevertheless, the ice sheet carved the  land deep enough to create the Puget Sound, an estuary between the Straight of Juan de Fuca and the rivers from the Cascades and Olympics. Many of the hills in the lower region are actually drumlins, oriented south to north, the were formed during the retreat of the ice sheet. The UW is on one of these drumlins. Geology continues to shape the region through the ongoing volcanism, glacial accumulation and ablation (many consider glaciers a sub-field), and the occasional earthquake.

Cool animation:
http://www.youtube.com/watch?v=YHWMHzi_deg



Just 100 years ago, most of Union Bay Natural Area was underwater. The surrounding area was not that developed and what was above water at UBNA was a marshland for Lake Washington. Some of the peat deposits from that time can still be seen today. After the ship canal opened in 1916, the water level in Lake Washington dropped 11ft, exposing much of present day UBNA to be open to emigrant wetland species. For 40 years, between 1926 and 1966, the city and Seattle and the UW used UBNA as the Montlake Landfill, probably because it was a convenient geographic low. After 1966, the landfill was closed and graded. In 1972 the area was declared a natural area. Restoration work began and the UW used the land to study how ecosystems develop and how species interact, despite its tarnished history. To this day, UBNA is used as a bird habitat, and educational center and a park for the surrounding community.

For further reading on UBNA:

http://depts.washington.edu/ubna/

http://en.wikipedia.org/wiki/Union_Bay_Natural_Area