The sandstone north bluff of Pinoy Hill

From the south beach of Seward Park’s Bailey Peninsula, the panorama is dominated by Mount Rainier and Lake Washington. Mount Rainier is a reminder that Seward Park owes its existence to the Cascade volcanos, which began erupting some 40 million years ago and through erosion supplied the bedrock that makes up the Bailey Peninsula.

Lake Washington and the current shape of Bailey Peninsula owe their existence to a different era: the ice age that ended only 14,000 years ago and which was the dominant force in shaping the current landscape. The peninsula shoreline illustrates the geologic impact of humans through activities such as building canals and roads.

A LIDAR image of Seward Park. Courtesy of Puget Sound LIDAR Consortium, via Dana Hunter
Tb: Blakely Formation from the Oligocene epoch. Medium-grain sandstone, coarse-grained sandstone, conglomerate, and minor siltstone, fresh to highly weathered, fossiliferous. Massive to well bedded. Deposited in a shallow marine near-shore sandy shelf environment. Distinguished from rocks of underlying Puget Group by presence of marine fossils and absence of volcanic flow rocks and breccias. Ql: lake deposits from the Holocene epoch. Silt and clay with local sand layers, peat, and other organic sediments, deposited in slow-flowing water. Most mapped areas are lake bottom sediments exposed by the lowering of Lake Washington in 1916. Qvt: Vashon till from the Pleistocene epoch. Compact diamict of silt, sand and subrounded to well-rounded gravel, glacially transported and deposited under ice. Commonly fractured and has intercalated sand lenses. Generally forms undulating, elongated surfaces. Visuals and information courtesy of USGS:

Plate Movement

The crust of the earth is broken into a number of large “plates” that are in motion relative to each other. Washington is part of the North American Plate. In the Pacific Northwest the small Juan de Fuca Plate lies offshore of the continent from northern California to southern British Columbia, and is traveling northeast at a rate of about 40 mm per year relative to the large North American plate. Where the two plates come in contact, the heavy basalt of the oceanic crust of the Juan de Fuca Plate sinks beneath the lighter continental rocks of the North American Plate. This process of sinking is known as subduction. The basalt crust angles beneath the continent and melts when it reaches a depth of about 60 miles. The melting basalt generates the magma that feeds the Cascade volcanos.

The Cascades began forming about 35 or 40 million years ago and have continued erupting to the present with one major gap between 17 and 12 million years ago. The present peaks are young; Mt. Rainier is probably not more than 1 million years old.

Juan de Fuca Plate and North American Plate
Image modified by National Park Service, originally from ÔÇťOregon’s Island in the Sky: Geology Road Guide to Marys Peak, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017

Seattle Bedrock

East of the Seward Park Art Studio soft sandstones make up the north bluff of Pinoy Hill. Harder conglomerates are exposed slightly to the north, where they project into Andrews Bay in the small promontory of Kingfisher Point near the dock storage area. These outcrops, along with outcrops at Alki Point and on the west side of Beacon Hill, are the only bedrock exposures in Seattle, which is mostly covered in glacial deposits. The rocks at all these sites are part of the Blakeley formation, dated to the Oligocene epoch some 26-37 million years ago by its marine mollusk fossils. At that time the Puget Lowland was part of the continental shelf. The Blakeley formation is made up of materials eroded onto the shelf from the growing Cascade volcanos to the east.

Pinoy Hill sandstone bluffs viewed from the south

Exposed bedrock at Kingfisher Point

Although western Washington was once part of the ocean floor, it became attached to the North American Plate and has been raised by the piling up of rock forced under it in the subduction trench of the Juan de Fuca Plate. Thus the basalts of the Crescent formation, which underlie most of western Washington (including Seward Park) and which outcrop in the Olympic mountains, were once oceanic crust. The sedimentary rocks of the Blakeley Formation sit on top of the Crescent basalts.

The Seattle Fault Zone

Why is bedrock only found in south Seattle? To the north of Seward Park the Seattle Fault runs just south of the I-90 floating bridge. North of the fault is the Seattle Basin. The same bedrock is found beneath the basin, but dropped down about 6 miles. The fault may have been active for the last 40 million years. The Seattle Fault was discovered in the late 1980s when it was realized that several different observations all indicated that a 20 ft uplift along this fault occurred 1100 years ago. This earthquake caused three different sections of forest to slide upright into Lake Washington. Two of these “sunken forests” slid off Mercer Island, one roughly across from the Seward Park fish hatchery and the other at the south end of the island. The sunken forests were discovered when Lake Washington was lowered in 1916. The timber was in good enough condition to be “logged” after standing in the lake for 1000 years.

The Seattle Fault is part of a fault zone about 3 miles wide that encompasses Seward Park. The north end of the peninsula and the bedrock outcrops may both mark the positions of minor faults in this zone. The faults indicate a north-south compression of this region.

Why is western Washington being compressed? Although the part of the Pacific Ocean that lies offshore from western. Washington rests on the Juan de Fuca Plate, most of the Pacific lies over the Pacific plate, which is slowly moving northwest relative to the North American plate. As the Pacific Plate slides northward along the San Andreas Fault in California, it drags the edge of California northward, rotating western Oregon and squeezing western Washington up against the more stationary rocks of British Columbia. Combined with the northeast movement of the Juan de Fuca Plate, this motion causes compression and thrust -faulting in the Seattle area in the north-south direction.


Seattle has been covered by glaciers flowing southward out of British Columbia at least six times in the last two million years. Seattle’s long north-south hills (drumlins), such as the Bailey Peninsula and Mercer Island, were shaped by glacial scouring and subglacial stream erosion, as were the troughs underlying Lake Washington, Lake Sammamish and Puget Sound.

The most recent ice sheet, the Vashon Stade of the Fraser glaciation, reached Seattle about 15,000 years ago. It advanced as far as Olympia, then rapidly retreated about 13,500 years ago. At its maximum it was about 3,000 ft thick. When the ice advanced past the Strait of Juan de Fuca, it blocked the sea level outlet, causing a lake to form ahead of the glacier. The lake and meltwater from the glacier both create characteristic deposits known collectively as advance outwash. These proglacial outwash deposits filled the Puget Lowland to an elevation of over 400 ft, as seen by the relatively uniform maximum height of local hills. Drainage from the glacier was through the Chehalis River Valley to Gray’s Harbor.

The Bailey Peninsula (and most of Seattle) is blanketed with glacial till or “hardpan”, a mix of silt, sand, gravel and clay with occasional cobbles and boulders that was deposited under or in contact with the ice sheet. Much of this material was transported long distances: the gravels seen along the lakeshore are more likely to contain glacially transported rock from British Columbia than local bedrock. The same transport explains the scattered larger rocks (glacial erratics) seen on the northeast shore and other places in the park. Several such erratics are visible across Andrews Bay at Ferdinand Street Park. Major troughs in the Puget Lowland such as the Lake Washington basin were probably carved by subglacial meltwater channels. Drumlins such as the Bailey Peninsula were probably formed under the edge of the ice as it retreated. Lake and meltwater deposits similar to those of the advance outwash formed as the ice receded, but these recessional outwash deposits are much smaller than the advance outwash and are confined mainly to the troughs.

Glacial erratics, boulders deposited by a retreating glacier

Lake Washington

Once the ice retreated past the Strait of Juan de Fuca, saltwater invaded not only Puget Sound, but also the Duwamish River Valley and Lake Washington. Deposits from the Cedar River soon cut off the connecting passage from Lake Washington to the Duwamish Valley, however, and Lake Washington became a freshwater lake. The Duwamish Valley remained an arm of Puget Sound until after the Osceola mudflow from Mt. Rainier 4800 years ago was eroded by the White River and redeposited in the valley, raising its floor above sea level.

For over 13,000 years Lake Washington drained via the Black River in Renton. The Cedar River joined the Black River 0.5 mi from the lake, and the Black joined the White River (now the Green River) to form the Duwamish, emptying into Elliot Bay. In 1916, the Montlake Cut of the Lake Washington Ship Canal was opened, lowering the lake 9 ft and rerouting its drainage into the ship canal. The Cedar River was also rerouted into the lake, so that the Black River now exists only as a small creek. Lowering the lake added 70 acres of shoreland to Seward Park, and much of the new shoreland on south Lake Washington was also kept as parkland along Lake Washington Boulevard.

Additional alterations to the shores of Seward Park have occurred. The wetlands that occupied the isthmus of Bailey Peninsula were filled. The construction of the loop road required grading the shores and importing large quantities of rock for the roadbed. Ironically, more visitors probably see this exotic rock ringing the peninsula than see the native bedrock.

Earthquake Monitoring

The bedrock of Seward Park and its position in the Seattle Fault Zone make it a good location for monitoring earthquakes. The Pacific Northwest Seismograph Network, centered at the University of Washington’s Geophysics Program, maintains both a broad-band velocity sensor and a 3-component accelerometer for detecting strong motion in the park. These instruments help geologists locate earthquakes and measure their motion, and help in assessing earthquake hazards.

Seismograph of the 6.8 February 28, 2001 Nisqually earthquake from the seismic station in Seward Park.