Too High and Too Steep

Reshaping Seattle's Topography

David B. Williams

• Publisher: University of Washington Press
• Year: 2015
• eBook ISBN: 9780295806181
• Topic: History
• Subtopic: Urban Planning & Landscaping
• Index: History

Residents and visitors in today's Seattle would barely recognize the landscape that its founding settlers first encountered. As the city grew, its leaders and inhabitants dramatically altered its topography to accommodate their changing visions. In Too High and Too Steep, David B. Williams uses his deep knowledge of Seattle, scientific background, and extensive research and interviews to illuminate the physical challenges and sometimes startling hubris of these large-scale transformations, from the filling in of the Duwamish tideflats to the massive regrading project that pared down Denny Hill. In the course of telling this fascinating story, Williams helps readers find visible traces of the city's former landscape and better understand Seattle as a place that has been radically reshaped. Watch the trailer: https://www.youtube.com/watch?v=af51FU8hHLI Too High and Too Steep was made possible in part by a grant from 4Culture's Heritage Program.

Sunrise over the Cascade mountains. Courtesy Chris Lewis.

1

Geology

IN LATE 2013, I FLEW BACK TO THE PLEISTOCENE. I DIDN’T EXPECT to do so when I got on a flight to Tucson, but soon after taking off from fogbound Sea-Tac Airport, I felt like we had time-traveled back seventeen thousand years. Emerging from the white shroud into crystal blue skies, I could see a sheet of fluff spread below me. Seattle had disappeared, Bellevue too, as had Tacoma, the Kitsap Peninsula, and the islands of Puget Sound. The clouds had erased all traces of humanity. Only the peaks of the Cascade and Olympic mountains popped above the whiteness. For the first time in my life, I could imagine what the landscape must have looked like when a glacier three thousand feet thick covered the Puget Lowland.

Geologists refer to that glacier as the Puget lobe of the Cordilleran Ice Sheet, one of the two great ice masses that covered North America during what is popularly known as the last ice age, running from about thirty thousand to ten thousand years ago. Without a doubt, the tongue of ice that crept down from Canada was the most important factor in forging the landscape of Seattle.

The ice, or rather water beneath it, gave us Lake Washington, Lake Union, and Lake Sammamish. The ice left behind frozen blocks that melted into Bitter Lake, Green Lake, and Haller Lake. It carved the city’s hills, ridges, and valleys; deposited the sediments that generate our landslides; and led to the formation of Puget Sound and the Duwamish River valley. Without the ice, we wouldn’t have our challenging winter driving conditions, constricted traffic corridor, hard-to-dig soils, pulse-raising downtown street climbs, or stunning views. Without the ice, Seattle would be flat and boring, though much easier to navigate.

The first big impact of the Puget lobe came when the glacier blocked the Strait of Juan de Fuca. Without a connection to the Pacific Ocean, water melting from the ice front flowed into an immense lake that spread from the Olympics to the Cascades. The ice-melt streams carried vast quantities of fine sediment, which settled in the bottom of the lake. If you have ever been to the mouth of a modern glacier and have seen milky white, sediment-choked water gushing out of the ice, you have seen this phenomenon, a by-product of the glacier’s grinding the land to powder. In the Seattle area, the lake deposits form a layer up to a hundred feet thick known as the Lawton Clay.

The best place to see the clay and silt in Seattle is where it was named, Discovery Park, formerly known as Fort Lawton. Along the base of the cliff just south of West Point you can find clumps of dark-gray to blue-gray, finely layered clay and silt, which have washed down from above. It is a wonderfully smooth and dense material that looks ideal for a mud bath. It is also ideal for impressing your friends, because you can pick up what looks like a fairly solid rock and break it in half. If you do so, look at the chunks in your hands. You will see that they are densely packed, like fudge. If you pour water on the clay pieces, very little will soak in, a characteristic that has had monumental implications for landslides and the topography of Seattle.

As the ice got closer to Seattle, the sediment coming out of the glacier’s snout changed. It became sandy and gravelly, evidence of deposition by high-energy streams. Known generically as advance outwash and specifically in our region as the Esperance Sand, this type of deposit is a common feature of glaciated landscapes. In Seattle, the Esperance is up to two hundred feet thick and lies directly on the Lawton Clay, a relationship clearly visible at Discovery Park. The Lawton is the dark-gray layer sitting on older, tan-to-brown interglacial sediments and is positioned under the younger, light-gray outwash beds.¹

If you turn around and look west, out over the sound, you can see another manifestation of the layering of the Lawton Clay and Esperance Sand, or what geologist Derek Booth has labeled the “most prominent single landform of the entire region.” He first appreciated this phenomenon during a meal at the Athenian Inn restaurant in the Pike Place Market.²

“I was gazing out the window at breakfast, when I noticed how all the hills line up. Of course, I had noticed this before, but I distinctly remember having an aha moment,” he says. “At the time, I had the advantage of working in these areas: I knew what was under the hills and I knew that they couldn’t have been planed down. They had to have been built up by the outwash.”

What Booth observed is something probably many have noticed from any point where one has a wide view of Puget Sound, such as the Space Needle or the Bainbridge Island ferry. All the Seattle hills, the islands of Puget Sound, and the hills of the Kitsap Peninsula top out at the same elevation, four hundred to five hundred feet above sea level. In other words, just before the Puget lobe of the Cordilleran Ice Sheet arrived in Seattle 17,400 years ago, the area between the Olympics and the Cascades was covered in a level plain made of the Esperance Sand atop Lawton Clay. Only a few features, such as Green and Gold Mountains on the Kitsap Peninsula and the Newcastle Hills, Tiger Mountain, and Cougar Mountain, rose above the level plain, or what is now the tops of the hills.

Finally, the towering Puget lobe—as high as six stacked Space Needles—arrived and deposited a mantle of sand, cobbles, gravel, and boulders atop the Kansas-like plain of Esperance Sand and Lawton Clay. Best known to urban dwellers as hardpan, and to geologists as Vashon till, this mantle, the youngest of the glacial deposits, is commonly only a few to ten feet thick, with some spots as deep as thirty feet. Geologists describe it as the material “carried and ‘smeared’ along the sole of the glacial ice.”³ The ice then continued to move over the landscape for another hundred miles south, as far as the city of Olympia, each year covering a stretch equal in length to about five to six football fields. Ice remained at the Puget lobe’s southern terminus for a time before melting, and as it melted, the terminus retreated northward. It passed back through Seattle around 16,400 years ago.

The long-term cover of ice in Seattle had three effects on the landscape. The most significant is the carving of the troughs that we call Hood Canal, Lake Sammamish, Lake Washington, and Puget Sound. The culprit, however, was not the ice, says Booth. Geologists had long hypothesized that the rasping tongue of the Puget lobe was the primary shaper of the Puget Sound topography; but starting in the early 1990s, Booth and fellow geologist Kathy Troost proposed what, to nongeologists, may be a counterintuitive idea. They suggested that rivers flowing under the ice—what geologists call subglacial water—had cut down into the relatively unconsolidated sand and clay and created those troughs.⁴

FIG. 1.1. GLACIAL EXTENT. The three-thousand-foot-thick Puget lobe glacier reached as far south as Olympia and began to melt back, or retreat, through Seattle about 16,400 years ago. The Olympic and Cascade mountains restricted its route through the area, creating hills in Seattle that trend north-south. Courtesy Washington Geological Survey, modified from Patrick Pringle, Roadside Geology of Mount Rainier Park and Vicinity, Washington Division of Geology and Earth Sciences Information Circular 107 (Olympia: Washington State Department of Natural Resources, 2008).

There really is no other way to carve Hood Canal, Puget Sound, and the two lakes, which have depths as great as thirteen hundred feet below the outwash plain. A look at the topography of these troughs would tell you that they are too squiggly to be cut by glacial scouring; ice is too stiff to do this, says Booth. In addition, if the ice had encountered the glacial lake in Puget Sound, the ice would have floated atop the water and had little effect on the landscape.⁵

Second, the ice acted like fingers gently running through sand, leaving behind a series of parallel valleys and ridges, or what we locals call hills. The best known of these are the famed seven that, according to local legend, Seattle was built upon. In order of highest to lowest elevation, they are Queen Anne (470 feet), Capitol (464 feet), Renton (412 feet), Beacon (364 feet), First (360 feet), Profanity (319 feet), and Denny (240 feet).⁶ In recent times, Magnolia (392 feet) and West Seattle (520 feet) have replaced Renton and Profanity in the city’s pantheon.⁷

The best way to experience the topographic effects of the glacier is to bike across Seattle. Traveling north or south is relatively easy as you follow one of the city’s many ridges or valleys, whereas riding east or west tends to be more challenging as you ascend and descend hill and dale. In case you wondered, Capitol Hill and Queen Anne have the steepest streets, though Denny Hill also had its fair share; nearly all run, or ran, east-west.

Finally, the three-thousand-foot-thick ice sheet compressed and consolidated the underlying sediments, making them denser and harder. “Compaction completely changed the engineering characteristics of the sediments,” says Bill Laprade, a geologist with the geotechnical firm Shannon and Wilson. Because the till is made of different grain sizes, it became the densest of the glacial sediments as the smaller particles filled in any voids between larger grains. In order to work it, contractors need special, toothed excavation equipment known as rippers. In contrast, contractors need only backhoes to dig the Esperance and Lawton beds. The sand is particularly easy to excavate because it lacks cohesion. The clay is more challenging because the fine-grained particles adhere to each other, making the Lawton stickier. Laprade says that the Lawton can also develop incipient cracks; and when the pressure is released, as in an excavation, large blocks can peel off, which is why contractors have to reinforce excavations for buildings.⁸

The dearth of level ground following the Puget lobe’s retreat back to Canada is a landscape feature that I return to throughout the book. The region’s glacial history left the would-be city with a deficit of flat land: there just wasn’t much nonhilly land anywhere. In an era when large beasts supplied most of the energy to move goods on land, hills did not make a transportation-friendly city. Later, as rail began to dominate, the steep slopes and high bluffs that descended to Elliott Bay made access to the city challenging.

Although today we have left behind the era of trains and become reliant on the car, we still face many glacier-induced challenges. Cars may be more flexible in route choice than fixed rails are, and may go up hills more readily than horses, but we cannot ignore our topography. Ever try to drive across Seattle in a snowstorm or start from a dead stop on a steep hill in a manual car? Planners cannot send roads just anywhere they deem necessary for more expedient travel, which is part of the reason why the urban blight we call the Alaskan Way Viaduct was built where it was and why we are taking it down.

The glacial processes I have been writing about have one thing in common. They operate at the slow pace that many define as the hallmark of geology. The next big change to hit the region was anything but slow. Known as the Osceola Mudflow, or Osceola lahar, it was a truly catastrophic event, one that caused perhaps the biggest single-day transformation of the regional landscape in recent geologic time.

It took place fifty-six hundred years ago following a volcanic explosion that blew off the summit of Mount Rainier, then more than sixteen thousand feet high. The eruption triggered a cascade of water, ice, and rock that plunged down the northeast side of the mountain at speeds in excess of 130 miles per hour. Still churning at 40 miles per hour more than twenty-five miles away, the Osceola continued to grow as it incorporated huge boulders, downed forests (which the debris had leveled), and previously deposited material in its path. Finally, about two hours and seventy-five miles later, the muddy stew slammed into what is now Puget Sound, where it had enough punch left to spread the remains of Mount Rainier for many miles underwater. During that single day, the lahar moved 4.9 billion cubic yards of material and altered more than 210 square miles, leaving deposits up to a hundred feet thick. As a comparison, the Panama Canal—often used as the measuring stick of modern earth-moving projects—took fifteen years to build and entailed moving an estimated 310 million cubic yards of dirt. Although much smaller than Osceola, the Panama Canal project still moved almost four times as much dirt as was moved in Seattle’s regrade and tideflat-filling projects.⁹

Although the Osceola did not reach Seattle, it and several subsequent lahars had a profound effect on the landscape. Before the Osceola event, what we now call the Duwamish River valley was one of the deep troughs carved by subglacial water; instead of land, it was a saltwater bay that extended twenty miles south, to what is now Auburn. When the Osceola hit, the mud and muck that flowed down from Mount Rainier began to accumulate in the trough. Because the deposits were poorly consolidated, the ancestral Green River was able to erode the lahar deposits and transport more sediment into the trough. Then another lahar hit. And another and another, each dumping material into the Duwamish trough, filling it in to create a slightly sloping plain carrying the Duwamish and Green Rivers. By the time the last lahar hit, about eleven hundred years ago, the repeated flows had pushed the mouth of the Duwamish to its present location.

Around the same time, another catastrophic event hit, when a magnitude 7 earthquake thrust the ground at the Duwamish delta up by twenty feet. The quake was generated by the most recent movement on the Seattle Fault, a subsurface tear in the earth that runs east-west for twenty-five miles, from Bainbridge Island through the southern end of downtown Seattle—about where the two stadiums sit—and out east beneath Lake Washington and Lake Sammamish. Conveniently, we can blame California for this geologic problem.

For the past ten to fifteen million years, tectonic plate movements have pushed the Sierra Nevada northwest by a bit less than half an inch a year. The Sierras act like a giant block and butt into the hard mass of rock encompassing the Coast Range of Oregon, which in turn advances slightly to the north and pushes into Washington. But western Washington cannot budge, because it runs into Canada, which is part of a very stable landmass. Seattle literally sits between a rock and a hard place as its subterranean innards are squeezed by a tectonic vise.

One good place to see the long-term effects of Seattle’s squeeze is a pair of anomalous mounds south of Boeing Field. You can see them best from the southbo...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Contents
• List of Illustrations
• Preface
• Acknowledgments
• Time Line of Topographic Events in Seattle
• Introduction
• 1 Geology
• 2 Seattle’s Historic Downtown Shoreline
• 3 Filling in the Duwamish River Tideflats
• 4 Replumbing the Lakes
• 5 Regrading Denny Hill
• 6 We Shape the Land and the Land Shapes Us
• Appendix: Volume of Dirt Moved in Seattle via Topographic Reshaping
• Notes
• Index