Geology 101

And now for a history lesson at this station! Union Bay Natural Area used to be a part of the Lake Washington (see picture below), completely underwater. Even before that, this area was carved out by the Puget Lobe of the Cordilleran Ice Sheet (1). Imagine this area under 3000 feet of snow! Underneath the snow were outwash plain deposits that exist because streams derived from the glacial front carried sediment and deposited the sediment at the toe of the glacier.

Outwash Plain Deposits

As the glacier advanced, the outwash plain deposits would be buried underneath. As the Puget Lobe receded, a glacial lake called Lake Russell was formed and underneath, the outwash plain deposits lay buried, ready for growth. Clearly, glaciation has resulted in lasting impacts on Union Bay Natural Area. In 1916, the Lake Washington Ship Canal was constructed, which lowered the water level of Lake Washington(2). As a result, the outwash plain deposits at the bottom of the lake became exposed at shallower areas, including Union Bay Natural Area. This fertile soil was subsequently colonized by many of the wetland species you learned about in station #3 (see picture on right) and formed a marshland. It was not a smooth transition from exposed soil to wetland habitat, look at the tab for "land-use history" for more information.

Sources:
1. Troost, K G, and D B. Booth. "Geology of Seattle and the Seattle Area, Washington."Reviews in Engineering Geology. 20 (2008): 1-36. Print.

2. Center for Urban Horticulture. Union Bay Natural Area. University of Washington, Seattle. Web. 4 June 2013.