Geology
Local Geology
The geology of Puget Sound is the product of millions of years of earthquakes, tectonic folding, and volcanic activity, and more recent sculpting by glaciers, streams, and rivers. The Olympic Mountains are accumulating sediment scraped off the Juan de Fuca Plate as it slides (subducts) beneath the North American Plate. Magma rising from melting of the subducting Juan de Fuca Plate generates the Cascade Range volcanoes, which have been erupting for 35 million years. The Puget Sound Lowland has been filling with sediment eroded from these two mountain ranges for millions of years.
Cascadia plate tectonics and earthquake sources (USGS).
Glaciers
At least seven glacial advances occurred in the Puget Sound Lowland over the last two million years. The last glaciation occurred about 17,000 years ago and covered the Shoreline area with 3,500 feet of ice before the glacier quickly retreated at a rate of 500 meters per year.
The glaciers extensively reshaped the land and evidence of the most recent glaciation can be found throughout Shoreline and the surrounding area as: drumlins, kettle lakes, and erratics (see below).
The most recent glacier also deposited a layer of sediment (called Vashon till) throughout most of Shoreline that contains a mixture of clay, silt, sand, and gravel. The very dense till does not absorb water well and is difficult to dig into. This poorly draining till causes temporary ponds to form after a heavy rainstorm. It also caused extensive peat bogs to form within shallow depressions between drumlins (see below) after the glacier receded. The ponds we see today at Ronald Bog and Twin Ponds Park are the result of peat mining in the mid-1900s which created open pits that filled with groundwater.
Contours show approximate ice thickness in feet at glacial maximum during the most recent glaciation. Map derived from ice thickness contours by R. M. Thorson (1980). Image courtesy of Washington Department of Natural Resources.
Drumlins
Ice advancing south deposited hundreds of feet of sediment and reshaped this material into elongated troughs and ridges called “drumlins.” Drumlins are oriented in the direction of ice flow, which was north to south. This is why it can feel like a roller coaster as you travel east-west across Shoreline. Examples of drumlins in Shoreline include Ashworth Avenue N in the Parkwood and Meridian Park neighborhoods, 5th Avenue NE in the Ridgecrest neighborhood, and NE Perkins Way at 11th Avenue NE. A lidar image of exceptional drumlins near Hood Canal can be seen here.
Kettle Lakes
Many of the small lakes such as Green, Haller, and Bitter Lakes in Seattle, and possibly Echo Lake in Shoreline, were formed from blocks of ice that detached from the receding glacier and became partially or completely buried by sediment transported by meltwater streams. These “kettle” lakes formed as the blocks of ice gradually melted and their surrounding sediments collapsed, leaving behind a depression that filled with groundwater. Shallow kettles with less groundwater sometimes become peat bogs instead.
Erratics
Other evidence of glaciation includes large boulders or “erratics” deposited by the melting ice. These include granite and metamorphic rocks that hitched a ride on the glacier from as far away as British Columbia. You might see these erratics exposed in drumlins, deposited on Puget Sound beaches below eroding bluffs, or resting at the bottom of ravines that were carved into glacial deposits.
Further info about erratics and a comprehensive list of erratics in the area can be found here. The largest erratic in Washington is in Lake Stevens and a number of large erratics can be found around Whidbey Island.
The 8-meter-tall Wedgwood erratic is located at NE 72nd St and 28th Ave NE in Seattle. It marks the eastern edge of an extensive deposit of rocks by the glacier.
Earthquakes
The Puget Sound Lowland is shortening in the north-south direction which causes east-southeast oriented faults that periodically produce magnitude 7 earthquakes. Shoreline is located between two regional fault zones - the Seattle Fault Zone and the Southern Whidbey Island Fault Zone. The last earthquake on the Seattle Fault occurred about 1,100 years ago and caused land on the south side of the fault to rise 35 feet relative to land to the north.
Earthquakes also occur along the Cascadia subduction zone off the coast of the Pacific Northwest where the Juan de Fuca Plate is sliding under the North American Plate. This subduction zone can create magnitude 8-9 earthquakes about every 500 years. The last earthquake along this zone occurred on January 26, 1700, and created a tsunami that reached Japan. This was just one of many tsunamis in the Pacific Ocean and Puget Sound caused by earthquakes in our region.
Thornton Creek
Thornton Creek once flowed freely through a forest and interconnected wetlands until urbanization in the early 1900s routed long segments of the creek into underground pipes. Before Interstate 5 (I-5) was built in the 1960s, the North Fork of Thornton Creek flowed openly through a wetland near where I-5 now intersects 145th St, and people occasionally caught steelhead trout there. The creek also supported coho salmon, sea-run cutthroat trout, and resident trout. During construction of I-5, two 2000-foot-long, six-foot culverts (see image to the right) were built for Thornton Creek to pass underneath the freeway before continuing its journey to Lake Washington through the parks and backyards of Northeast Seattle. The culverts are almost impassable for fish, so their construction ended the era of steelhead fishing in Parkwood. Efforts are underway to replace the culverts under I-5 and remove other downstream fish barriers to someday allow the return of native fish species to the upper watershed.
The ponds of Twin Ponds Park are fed by both Thornton and Meridian Creeks. The North Fork of Thornton Creek begins near Cromwell Park, passes through Ronald Bog, and continues south into the northern pond. Meridian Creek begins near Meridian Park and flows mainly through pipes before entering the west side of Twin Ponds Park and draining into the southern pond. The ponds drain to the east via Thornton Creek, which passes through the I-5 culverts and continues 4.5 miles until it reaches Lake Washington.
