At Discovery Park, on Puget Sound in Seattle, a high bluff (sea cliff) shows a record of events that mark the coming of the Vashon glacier that filled the Puget Sound region. The base of the bluff in this photo is composed of sedimentary beds. At high tide, the waves of Puget Sound lap up against this outcrop.
The layers of sediment in this picture record the Olympia Interglacial interval, the time before the approach of the Vashon glacier. During the interglacial, meandering streams and rivers flowed north toward the sea, across what is now the Puget Sound region. The beds in the picture are evidence of those rivers, with ripples in the beds of sand indicating water flowing in broad, shallow stream channels; silt-plus-clay-rich layers indicating ponds and backwaters; and woody debris indicating trees, bushes and small plants growing in the area.
The Olympia Interglacial beds at the top of this picture are partly covered by blue-gray clay that has flowed down from the next layer of sediment above. Higher in the bluffs, above these Olympia Interglacial beds, is a sequence of sediments that records the approach and arrival of the Vashon glacier.
Possession Formation, Lawton Clay, Esperance Sand
This picture shows the Olympia Interglacial beds up to the break in slope, the point at which the slope becomes less steep and where many tufts of grass and a lone, dying tree grow. That zone where the grass and trees grow is where the Lawton Clay is located. The Lawton Clay, like most clay, is a weak, soft material that turns into slippery mud when wet. Select the image to see a larger view. Use your browser's back button to return to this page.
The Lawton Clay, like any clay that is deposited in broad, flat layers, indicates that a large body of low-energy water was present. Clay accumulates at the bottom of lakes or relatively stagnant bodies of water.
As it entered the northern part of the Puget Sound basin, the advancing Vashon glacier probably dammed the drainage of the rivers whose sediments are seen in the Olympia Interbeds. The dammed rivers formed a series of large lakes. The Lawton Clay accumulated at the bottom of these large lakes.
Above the Lawton Clay in the photograph are steeper cliffs of Esperance Sand. The Esperance Sand consists of beds of clean sand and gravel. Based on the nature of the sediments, and the cross-beds and other sedimentary structures in the Esperance Sand, geologists reason that it was deposited in braided streams of rapidly flowing water. This indicates that the Vashon glacier was adjacent to the area--a wall of advancing ice looming nearby to the north that shed rivers of melt water ahead of itself and deposited an apron of sand and gravel around its periphery.
Most landslides in the Seattle area occur when steep land on top of the Lawton Clay slides after heavy rains have saturated the ground with water. As rainfall accumulates and infiltrates into the ground, the Esperance Sand, which is porous (full of holes) and permeable (the holes are connected so water can channel through), fills up with water to the point that the water pressure is pushing the sand grains apart from each other, undermining the integrity of the sand. At the same time, the water cannot penetrate through the nearly impermeable Lawton Clay underneath the sand - which is why the water backs up in the sand and over-pressurizes the sand in the first place. The water that is able to penetrate slightly into the Lawton Clay turns it into, effectively, slick mud.
The combination of the added weight of water in the ground, the weakening of the Esperance Sand by the hydraulic pressure caused by the sand being saturated with water, and the weaking of the Lawton Clay underneath into slick, slippery mud that acts as a lubricant, are a "perfect storm" that sets off landsliding once enough rain has fallen and soaked into the ground behind Seattle's glacial-drift-filled cliffs and steep slopes.
This picture shows the Vashon Till, at the top of the bluff near West Point at Discovery Park. The Vashon Till records the presence of the Vashon glacier itself. Like all glacial tills, it is an unsorted, unbedded mixture of clay, silt, sand, cobbles and boulders. Closer study reveals that some of the boulders are faceted (have flattened sides), with striations on the facets. Striations are parallel lines or grooves that look like shallow scratches. These boulders were carried at the base of the flowing ice. They were pressed by the tremendous weight of the overlying ice, scraping against dirt and rocks beneath the glacier. This forceful scraping results in faceting and striations. No other process is known that can alter boulders in such a fashion--one of the many lines of evidence that this deposit is indeed glacial till. Select the image to see a larger view. Use your browser's back button to return to this page.
Puget Sound and Beach Erratics
This picture shows Puget Sound from Discovery Park, Seattle. The lowlands around Puget Sound are covered by glacial drift. The glacial sediments were smoothed by the ice at the base of the Vashon glacier into the relatively flat horizon you see on the hills across Puget Sound. Select the image to see a larger view. Use your browser's back button to return to this page.
The troughs of the Puget Sound lowland, including the main channel of Puget Sound shown in the picture, were excavated through a combination of abrasion and plucking of the substrate (the rock and sediment underneath it) by flowing glacial ice, and by hydraulic flushing and fluvial erosion (erosion by flowing water) due to the huge amounts of melt water that were forced from beneath the glacier. As a result of the hydraulic pressure from the weight of the glacier above, the subglacial melt water was forced to flow out from beneath the glacier. Where the ground beneath the glacier sloped upward in the down-pressure direction, the water was forced to flow uphill. This erosion by subglacial melt water, alternating with erosion by the direct contact of ice with the substrate, eroded the deep trenches that are now filled with salt water and called either Puget Sound or the Salish Sea.
Uplifted Platform on South Bainbridge Island - Evidence of a Major Earthquake on the Seattle Fault About 1,150 Years Ago
This photograph shows the southeast end of Bainbridge Island, near Restoration Point, across the water from Discovery Park. A flat, tan-colored terrace runs just above the shoreline, to the right of the sailboat. This is a wave cut terrace, which records where average sea level used to be. Select the image to see a larger view. Use your browser's back button to return to this page.
Approximately 1,150 years ago, a great earthquake on the Seattle fault abruptly uplifted this wave-cut terrace, according to a variety of geologic evidence. The same earthquake apparently uplifted a wave-cut terrace directly across Puget Sound at Alki Point and caused a tsunami to wash up across West Point at Discovery Park and other shores and estuaries of Puget Sound, such as Possession Point on Whidbey Island and the Snohomish River estuary in Everett. It also caused numerous landslides to come tumbling down steep slopes in the Puget Sound region.
In a few places, whole slabs of soil and intact, upright trees underwent landsliding into local lakes, including Lake Washington, leaving patches of "ghost forests" standing on the bottom of the lake, submerged beneath the water. Radiocarbon ages along with dendrochronology (based on studying annual growth rings in trees) finds ages overlapping in their error bars at around 1,100 yr BP (before the present era, which means before 1950 AD).
Evidence of a tsunami within Puget Sound comes from estuaries and marshes north of the Seattle Fault, for example the one at West Point, just a mile north of the fault. Coastal marshes form at sea level, with tide water filling their channels at high tide. The marsh at West Point was suddenly buried beneath a layer of sand that washed across it. At the same time, the level of the marsh, the elevation of the land, dropped below sea level. It took many years for the marsh to fill in with sediment and get re-established at sea level. The radiocarbon age of the dead plant material below the sand layer comes out around 1,100 years BP.
Therefore, according to the evidence, at approximately 1,150 years ago, north of the Seattle Fault, the land dropped a bit and was hit along the coast by a tsunami. South of the Seattle fault during the same event, the land was thrust upward abruptly, the wave-cut terrace on the south end of Bainbridge Island rising nearly 15 feet, the one at Alki Point in West Seattle 5-10 feet.
This sudden buckling and thrusting of the floor of Puget Sound during an earthquake ~1,150 years ago disrupted the landscape of the central Puget Sound region from Seattle all the way west to the Olympic Mountains and north to Whidbey Island. The earthquake generated an inside-Puget-Sound tsunami. The stress in the crust continues to strain the rock along the Seattle fault. A big earthquake happened there before, so it could happen there again.
Glossary terms that appear on this page: interglacial; silt; clay; varve; cross-bed; sedimentary structures; sand; gravel; braided stream; cobble; unbedded; unsorted; till; striation; wave-cut terrace; estuary
Virtual Field Site--Discovery Park
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes