Geology. The Seattle area sits on layers of interesting geological history unlike that found in other parts of the country. How the foundations of our landscape developed has far-reaching implications today, as evident by recent mudslides and other hazards.
"Solid ground" is beginning to seem a shaky term in Seattle.
From earthquake warnings to recent storm-caused landslides, the city's terra firma may seem anything but.
Actually, many of the underpinnings here are quite good. But Seattle's position on the West Coast's earthquake and volcano zone and its sculpting by Ice Age glaciers give it one of the most fascinating urban geologies in America.
The city's natural and human geologic history was nicely summarized in an August 1991 article in the Bulletin of the Association of Engineering Geologists, written by consulting engineering geologist Richard Galster and William Laprade of Shannon & Wilson. That summary forecast the kind of landslide problems recently observed.
By geologic standards, Seattle's landscape is very, very young. Just 14,000 years ago, the land the city sits on was still under 3,000 feet of ice, part of the Ice Age's titanic Vashon Glacier, which extended from Canada to south of Olympia. When the ice melted, sea level rose 300 feet and filled the trough the ice had carved, creating Puget Sound.
The region is still witnessing the erosion and settling that has followed that tumultuous episode.
When explorers and pioneers arrived, they found Elliott Bay a somewhat unpromising place. The water seemed too deep for easy anchoring, steep hills crowded the shoreline, and a vast salt marsh and tidal flat occupied the area where the Kingdome, city industrial area and Boeing Field are today.
Reshaping the landscape
Accordingly, the city has been radically transformed by engineering. Hills have been sluiced away, tidelands filled and rivers rerouted. The industrial area's Harbor Island is the largest artificial island in the United States.
Seattle "probably represents the most infilled, graded and leveled of all North American cities," Bulletin editor Allen Hatheway wrote.
Much of the Puget Sound basin, including Seattle, is marked by long, north-south trending ridges called drumlins that were deposited by glaciers.
Capitol Hill and Beacon Hill, for example, were once part of a single long ridge. The gap between the two that Interstate 90 traverses was cut from 1907 to 1912 to connect downtown to Rainier Valley. Engineers created a 90-foot-deep ravine by sluicing away 3 million cubic yards of soil to help fill the Kingdome area to the west.
In another piece of engineering, workers turned 100-foot-high Denny Hill into what is now Denny Regrade. The hill covered 62 city blocks but disappeared as 6 million cubic yards of earth were sluiced into Elliott Bay between 1903 and 1928.
That didn't come close to filling Seattle's harbor. Elliott Bay is so deep in places - 300 feet - that pioneers had to use a horseshoe on a clothesline in 1852 to find a place shallow enough to anchor, Galster and Laprade recount. The government's Wilkes Expedition said the place showed little promise as a harbor.
Seattle's original downtown was sited at Pioneer Square because it represented a scrap of relatively gentle hill next to a scrap of suitable anchorage. Tidelands to the south and waterfront along downtown to the north were filled in later.
When pioneers arrived, Lake Washington was 9 feet higher than the level of Lake Union, and Salmon Bay at Fisherman's Terminal was 10 feet lower. The Lake Washington Ship Canal was cut between 1911 and 1916 to bring the larger lake down to the smaller's level, creating today's lower Lake Washington shoreline. The Ballard dam and locks were built below Salmon Bay to raise it to match Lake Union.
During all this excavation, engineers discovered that the city roosts on the debris of the Ice Age, or the rubble left over from the Vashon Glaciation. And that was only the latest of at least five major glacial pulses over the city as the Ice Ages waxed and waned, covering Seattle's bedrock with as much as 3,700 feet of sand, clay and gravel.
Bedrock comes close to the surface of the city in only one place: the region from Seward Park west to Beacon Hill.
Far underneath Seattle, however, is one of the most peculiar bedrock formations in the United States. In less than a mile between Beacon Hill and downtown, bedrock dives more than 3,000 feet below sea level like a steep moutainside. Continuing north, it begins climbing back toward the surface, until it is only about 1,800 feet underground by the King-Snohomish County line.
This is such an uneven change in the rock crust that a person actually weighs slightly more (because the crust is thicker and gravity stronger) south of this break than north of it.
Geologists have theorized this break may be part of a fault - the famous Seattle Fault - that runs from Bainbridge Island to Lake Sammamish, under the vicinity of the Kingdome. This fault may be the source of a huge earthquake that shook Seattle 1,100 years ago. It also appears to line up with the Olympic-Wallowa lineament, a geologic boundary that runs from the Olympics across Seattle and the Cascades all the way through southeast Washington.
Sitting atop this geologic anomaly is a three-layer cake left by the Vashon Glacier, which was gone from the Puget Sound basin by 10,000 years ago.
At the bottom is Lawton Clay, which can range from a few feet to 100 feet in thickness. The clay is the sediment that formed at the bottom of a gigantic lake created in front of the glacier as the ice advanced southward and blocked the normal outlet of rivers.
The dam at the Ballard Locks is built atop Lawton Clay.
The clay includes boulders as big as 10 feet across. They were dropped into the prehistoric lake after being carried by icebergs that broke off from the glacier face and then melted.
Above this is Esperance Sand, a layer of sand and gravel washed out in advance of the grinding glacier as it overrode the lake. Although the boundary between the sand and clay layers is not always sharp, in heavy rainstorms water percolates through the sand and runs sideways on top of the denser clay, setting up the potential for a slide where the two meet.
Atop this is Vashon Till, a mess of cobbled rocks, dirt, clay and sand the glacier left as it advanced and retreated. Ranging from a few feet to 30 feet in thickness, it is the deposit most commonly encountered in Seattle. It creates the "hardpan" found just below the topsoil that is the curse of contractors or anyone sinking post holes.
Vashon Till is pretty firm stuff, having a consistency reminiscent of concrete. The engineers said vertical walls up to 50 feet high have been cut through this layer without collapse or erosion because the till is so solid, having been packed down by the weight of ice more than half a mile thick.
Most Seattle houses and buildings have been erected atop Vashon Till, a good platform in earthquakes and floods.
River valleys such as the Duwamish have a separate layer of alluvium deposits formed by rivers; the layer includes sand, gravel, silt, peat and organic debris up to 100 feet thick. Those valleys and tideland fill are a poorer foundation in an earthquake because their soil can shake apart and lose all strength.
Topography continues to change
Meanwhile, the moonscape of tumbled glacial debris left by Ice Age glaciers has been settling and eroding ever since.
The shoreline of Lake Washington, for example, is the site of numerous old landslides where steep hills have slid into the deep basin gouged out by the glaciers.
It was not until about 5,000 years ago that sea level - which was lowered when ice on land locked up so much water - rose to approximate its present level. (As the planet continues to warm, sea level is still rising slightly. The present rate at the Seattle tide gauge is 8.4 inches per century.)
Since the sea has come back, the bluffs of glacial till around Puget Sound have been retreating at a rate of about one to six feet per century, depending on the location. Recently this rate has been halted in most places in Seattle, however, either because of broad stable beaches or because of human improvements such as railroad embankments, bulkheads and seawalls.
An exception is Magnolia Bluff, where the West Point area around Discovery Park and Perkins Lane continues to experience erosion.
Slides are not the only problem in the Seattle area. The Newcastle Hills near Bellevue and the Renton region are dotted with old coal mines where most of the shafts were not backfilled. Sometimes they collapse, with 200 cases of subsidence since studies began in 1978. Newcastle Hills is also, however, an area where firm bedrock comes to or near the surface.
Other constant geologic threats here are earthquakes and the possibility of an eruption or mudslides at Mount Rainier. Scientists are studying both possibilities right now.
Will the city's population head for firmer ground?
Clearly Seattle's geology is not as stable as the eastern United States or England. But much of what makes the city so spectacular - its deep Sound, glittering lakes, glorious views and distant snow-capped volcanoes - are the product of its tumultuous geography. And that means more people, not fewer.