With the 30-year anniversary of Mount St. Helens’ legendary eruption just this week, we need no reminders of how much geology has shaped our region – even here in the city, as a recently updated map from the U.S. Geological Survey of the northeastern Seattle quadrant shows.
The map takes us back in a geologic time capsule for the areas now bordered roughly by Mercer Street in the south, Northeast 168th Street in the north, 15th Avenue West on the west and Lake Washington on the east. Basically, here’s what’s underneath Maple Leaf, which includes the third-highest hill in Seattle:
Till – Compact diamict of silt, sand and sub-rounded to well-rounded gravel, glacially transported and deposited under ice. Commonly is fractured and has intercalated sand lenses. Generally forms undulating, fluted surface; tends to drape topography and is found at both lowest and highest elevations in map area. Unit is typically 1 to 10 m (3.2 to 33 feet thick) and generally very dense. Uppermost meter generally is weathered and moderately dense; deeper levels characteristically are unweathered and very dense.
Although Seattle saw its last ice sheet about 16,600 years ago, multiple glaciers advancing from British Columbia in the past 2 million years helped shape the landscape we see now, spreading both glacial and nonglacial deposits over the irregular bedrock surface, the introduction says. More recently, faults and folds as little as 1,100 years ago further changed the landscape.
The map also identifies two main topographic areas of Northeast Seattle: an upland plateau in the north and the more hilly south end, separated by none other than the valleys of Thornton Creek, along with Pipers Creek and Green Lake. But here’s my favorite part of the map’s introduction:
Within the boundaries of the map area are two large urban lakes, including the most heavily visited park in the State of Washington (Green Lake Park); a stream (Thornton Creek) that still hosts anadromous salmon despite having its headwaters in a golfcourse and a shopping center … .
Thornton Creek, the little creek that could.
