you would not discover any glacial material. Instead, the black bedrock consists of the oldest rocks in Seattle, the 42- to 50-million-year-old Puget Group, a several-thousand-foot-thick layer cake of many separate rock formations.10 The rocks reached the surface because the tectonic vise has pushed the south side of the Seattle Fault up thousands of feet relative to the north side. These pimples along the Duwamish stand above their surroundings because the hard sandstone resisted glacial erosion. What makes the hills particularly interesting is that the Puget Group rocks played a crucial role in Seattle’s early economic and topographic history. Farther east, and forming parts of the Newcastle Hills and Cougar Mountain, are rich coal beds within thick layers of sandstone. The rocks of the Puget Group formed in rivers and swamps on a broad coastal plain. As younger sediments were deposited, they compacted the underlying swampy peat, slowly altering it to what would become the most valuable beds of coal on the West Coast. The coal was initially found at or near the surface, usually by someone searching for something else. Excavating it required tunnels, or gangways, that followed the veins deep into the hills. The beds were up to twenty feet thick, but typically much thinner and often separated by thick layers of sandstone. Although the miners still needed to dig gangways up to half a mile underground, they were able to reach the coal only because it too was pushed up to the surface by the Seattle Fault; had they desired to access the coal on the north side of the fault, it would have required drilling down thousands of feet. The best place to experience the most recent effects of the Seattle Fault is near the mouth of the Duwamish River at what is known as Terminal 107 Park, not the cleverest name but a place that has become one of the most famous geologic spots in the city. From the parking lot, a dirt trail leads through shrubs down to the river, and to a bank composed of sand, silt, and clam shells. Look carefully and you will see that many clams lie on their side with the bent part of their shell pointing 22 CHAPTER 1 EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use upward, which would have facilitated the use of their siphons for feeding and respiration. Geologists such as Brian Sherrod of the United States Geological Survey propose that the Seattle Fault killed these clams. He notes that, in contrast to their modern location well above high tide, these clams were intertidal organisms, meaning they lived between the high and low tide lines. Something must have raised them to their present elevation. The most likely scenario is uplift during an earthquake along the Seattle Fault, which killed them and left them in their former life-position. This is the magnitude 7 quake of eleven hundred years ago, the last recorded movement of the fault.11 If you take archaeologists to this same spot, they will tell you a different story: The clams are not in a life position but in a death position. They are part of an extensive garbage pile, or midden, made of shells, fish, mammal, and bird bones; stone and bone tools; and charcoal and other human debris. It developed over the centuries from materials deposited by Native people who lived along the Duwamish. The archaeologists recognize that this particular bank is a bit odd because it lacks the charcoal and fire-cracked rocks usually found with middens. Nor does it have the classic black, greasy look often found in middens, but studies show obvious midden deposits in other parts of the park. The archaeologists don’t dispute that the bank and the surroundings were uplifted; they just don’t think the riverbank is a clear-cut illustration of uplift as do geologists. There is one other feature to note at this location: the dark-gray sand atop the bank. Sherrod and others see it as evidence of the last of the four lahars to descend Mount Rainier. They cannot tell if the lahar, which occurred right around the same time as the last movement on the Seattle Fault, flowed all the way to this spot, or if the river carried the sediment here from a short distance away. Either scenario indicates that lahars from Mount Rainier have reached Seattle, carrying with them land- and life-changing quantities of soil and trees and rocks. That last lahar, combined with the last movement of the Seattle Fault, had a further effect on the Seattle landscape. When the fault lifted the land and the lahar deposits by twenty feet, it must have dammed the Duwamish and created a lake behind the uplifted delta. The lake probably lasted only a handful of weeks before the river eroded through the dam, says Sherrod, but it would have taken many decades Geology EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use 23 or even centuries to redistribute the uplifted lahar sediments across the delta of the Duwamish. In doing so, the river established a precedent of erosion and deposition that eventually led to the formation of the Duwamish River tideflats, one of the central geographic features in the early history of Seattle. In addition, Ralph Haugerud, a colleague of Sherrod’s at the Geological Survey, says the uplift made the Duwamish River valley better for farming.12 Previously, the valley bottom would have had soggy soils. After rising twenty feet or so, the soils would have been much better drained and much better for growing crops, a fact initially exploited by Native people and later by the valley’s other early settlers. No other site in Seattle is so exciting to Sherrod and his fellow geologists, for no other site provides such graphic evidence of the region’s intertwined history of earthquakes and volcanoes. In that way, the little bank of sand and clams at Terminal 107 is sobering. It clearly shows that we live in a landscape that was shaped by and will continue to be shaped by natural forces far beyond our control. We will do our best to counter those forces with good engineering and planning, but ultimately our lives will be changed the next time Mount Rainier sends a lahar our way or the Seattle Fault shifts the ground by twenty feet. Humbling as this location is, we can also thank the geologic forces that formed it for the development of the city. In Ralph Haugerud’s words: “Seattle is here because of the Seattle Fault.” Not only did the uplift lead to better soils in the Duwamish, but it also formed the landing site of the Denny party, the men, women, and children of Seattle’s founding families. On November 13, 1851, the schooner Exact dropped twenty-two people on the little bench of land that we call Alki Point. Led by Arthur Denny, they would overwinter at Alki before eventually moving across Elliott Bay to land near what is now Pioneer Square. That bench was and still is an unusual spot in Puget Sound. Not only did it have good wood for building, but also it was flat and above the high-tide line. Few other sites nearby had these features, and none in such an ideal combination as Alki. It was the perfect spot to land, and one long known by the Duwamish people. They called this raised bench Sbaqwábaqs, or Prairie Point, an indicator of its unusual nature and loca24 CHAPTER 1 EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use tion. Plant collections from the late 1800s—some of the earliest in the city—reveal that plants adapted to drier, more prairielike conditions thrived here. Haugerud says that we can further thank the Seattle Fault for the stellar deepwater harbor, which the earliest settlers needed in order to survive. When the Puget lobe encountered the Gold and Green Mountains, Newcastle Hills, and Tiger and Cougar Mountains, the high hills constricted the glacier in a tectonic girdle, which increased the pressure on the subglacial water and made it better able to excavate downward. The result is that the deepest part of Puget Sound is just north of the Seattle Fault. Had it been shallower, sediment could have filled it in more and led to a lower-quality harbor. Depending upon your worldview, and tolerance for risk, we are either fortunate or unfortunate that those early settlers didn’t realize the reasons the location seemed ideal. “If you look at all the major population centers in Puget Sound—Bellingham, Everett, Seattle, Tacoma, and Olympia—almost all of them have big, active, ugly faults under them,” says Sherrod. “We definitely would not pick those places today.” Geology may have led to the formation of our city, but it also has the potential for serious destruction, a fact that drives many of our modern land-use decisions. One other type of short-duration event has long played a role in shaping Seattle’s topography. To explain this phenomenon, I turn to someone who played his own prominent role in shaping Seattle. On February 27, 1897, Seattle city engineer Reginald Thomson sent a two-page letter to the city’s corporation counsel, or lead attorney. The subject was landslides, which Thomson wrote had occurred in Seattle “from a time to which the memory of man runneth not back.” The reason had to do with the interaction between a layer of impervious blue clay that lay at the base of the land and pervious glacial drift atop it. When it rains, wrote Thomson, water percolates through the drift “in devious ways” until it reaches the clay below, resulting in a “condition of saturation and suspension” that continues until “the surface ground breaks and settles down.”13 Using more recent terminology, a modern geologist would describe how the pressure of water that has penetrated through Geology EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use 25 Esperance Sand increases at the point of contact with Lawton Clay and makes the slope susceptible to sliding. Go to any steep hillside around the city and you will find evidence of landslides or future landslides. Look for areas mostly bare of shrubs or trees, such as the east side of Magnolia Hill (above the Magnolia Bridge), which slid in 1997; areas rich in springs and seeps, evidence that water has percolated down to the Lawton Clay and is following gravity to the surface; areas covered in alders and maples, trees that pioneer unstable terrain; and areas with stairs askew, which look as if they were made by a drunk contractor and indicate ground movement.14 The Seattle Landslide Study reported that more than thirteen hundred landslides had hit Seattle since 1890.15 These included high-bluff peeloffs, groundwater blowouts, deep-seated landslides, and skin slides. Skin slides, which are small and can be triggered by intense rainfall, are the most common. Deep-seated events are the most spectacular; one of the best known in recent years took place on Perkins Lane in the Magnolia neighborhood, when part of the bluff slid and carried several houses down to Puget Sound. The landslide occurred in January, by far the most likely month for slides, with almost three times more than the nextnastiest months, February and March. Not coincidentally, November, December, and January receive the most precipitation. If ice and water are the great sculptors of our landscape, then landslides are the fine chisels, nipping the rough edges and often leaving behind very steep, sometimes nearly vertical, slopes. Usually fairly small and localized, landslides reveal the weak spots in the landscape, the places one might not want to build. (Wave action along the city’s bluffs also helped generate places that early home builders avoided, such as the nearly vertical cliffs found at Discovery Park and north of Golden Gardens Park along Elliott Bay.) Landslides are one reason that green spaces such as Interlaken, Frink, Carkeek, and the Duwamish Greenbelt became parks and not homesites. But we have since rewritten the story line, ignoring the historic reluctance to build on clearly less-than-stable slopes. More than 80 percent of the recorded slides owe their slippage to human disturbance, including poor installation and poor maintenance of drainage systems, leaving broken pipes unrepaired, imprudent pruning of stabilizing vegetation, and the chopping off of the toes that protect slopes. This latter problem continues to exasperate hazard-prevention planners. Think of a mirror 26 CHAPTER 1 EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use resting on a hardwood floor and leaning against a wall. One easy way to stabilize it is to put a heavy object in front of the mirror. With the weight in place, the mirror doesn’t slide; without it, seven years of bad luck. The same concept applies when people remove material from the toe, or base, of a slope. Many of the landslides that have hit since 1890 would probably have occurred in geologic time; but with human activity, we have fashioned a new time scale, one based on human time. In doing so we have made landslides more relevant to the city’s topography and to those who live here. Other green spaces previously remained green because of another feature, the abundant streams that once meandered across the landscape. Nearly all have vanished under concrete or fill, but if you look at the survey maps produced by the General Land Office between 1858 and 1862, you will find dozens and dozens of waterways illustrated by the cartographers. Most started as the uncountable springs and seeps that emerged from the Esperance-Lawton contact. The features carved by the majority of these waterways were small scale, but they also gave texture to the landscape. Where once was a smooth, rounded hill, now there were furrows and gullies, clefts and dips, and perhaps a pond or two. Growing up at the north end of Capitol Hill, my friends and I had the ravine, a playground of creeks that had carved the hillside into a scalloped landscape perfect for kids to explore. It surprised me when I later learned that the place had the formal name of Interlaken Park. As with most other locales in Seattle, none of the creeks in the ravine had names, though there are more than forty-five officially designated creeks in the city. I suspect that few of those names are known outside the neighborhood where they flow. Probably the only well-known creek names are those of the biggest: Thornton, Longfellow, Ravenna, Taylor, and Fauntleroy. Each of these sizable streams also formed large topographic features on the landscape; some of the smaller streams created similar, but smaller, features. Part of what is intriguing about the stream-carved ravines and canyons is that some cut across the general grain of the glacial landscape. For example, both Ravenna and Thornton have long stretches that run northwest to southeast. Geologists do not have a good theory to explain these anomalous features. The streams could be flowing in channels incised by subglacial streams, or they could be following some unrecognized tectonic structures, which may account for Seattle’s Geology EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use 27 most significant northwest-southeast-trending topographic feature, the long diagonal now filled by the Lake Washington Ship Canal. It is the city’s biggest geological enigma. In few other major cities has such a set of dynamic geological forces been so integral to the life of the city. Seattle is not unique in being built on a glacial landscape—Boston, New York, Chicago, and the Twin Cities are situated in such landscapes, too. But it is unusual in sitting in a basin between two mountain ranges, which caused the ice to flow and erode differently. This region’s relative warmth also led to a dynamic between ice, water, and erosion that makes it differ from those locales. The result of these differences is that we have steep hills and other cities don’t. Nor is Seattle unique in sitting on a geologically vulnerable landscape. All of California’s major coastal cities must worry about earthquakes, and New Orleans has to plan for its subsiding waterfront and the rising sea levels. Certainly the locations of most cities have been dictated by geography, particularly by proximity to water, which is shaped by a location’s geologic history; but Seattle’s geology has produced a unique set of problems, from earthquakes to landslides to volcanic eruptions, that pervade our lives. In everything from the city’s founding to its earliest economic survival to its later commercial development, from where we live to where we play, from how we build infrastructure to how we disassemble the land, Seattle’s fate has been inextricably tied to geology. I don’t think we consciously recognize this. It is deeper than that. Only when large-scale events, such as earthquakes and volcanic eruptions, affect us do we fully admit to ourselves the forces that have shaped and continue to shape our lives in Seattle. But we also know subconsciously that we cannot escape these forces. Perhaps that is why we have so readily accepted the great changes we have wrought on the land. It is our past and it is our destiny. 28 CHAPTER 1 EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use EBSCOhost – printed on 5/11/2020 2:59 PM via SEATTLE CENTRAL COLLEGE. All use subject to https://www.ebsco.com/terms-of-use Concepts to Review: Igneous Rocks and Volcanoes New Vocabulary Rock cycle Extrusive igneous rock Intrusive igneous rock Partial melting Decompression melting Flux melting Mantle plume Bowen’s reaction series Mafic Felsic Fractional crystallization Porphyritic Aphanitic Phaneritic Vesicular Glassy Country rock Xenolith Pluton Batholith Sill Dike Laccolith Viscosity Effusive Explosive Cinder cone Composite/strato-volcano Shield volcano Igneous province Columnar jointing Pyroclastic Lahar Tephra Pumice Questions to Answer 1. 2. 3. 4. 5. What are the two primary ways a melt can form and where do these conditions exist? How is a felsic magma (or rock) different from a mafic magma (or rock)? What does the texture of a crystalline igneous rock tell you about its formation process? Which minerals melt first when a rock experiences partial melting? Using Bowen’s Reaction Series, explain how fractional crystallization changes the elemental and mineral content of a melt. 6. How does the amount of silica in a melt affect its viscosity? 7. Distinguish between dikes, plutons, sills and other instrusive igneous features Rocks to be familiar with gabbro, basalt, granite, diorite, rhyolite, andesite, pumice, tuff, obsidian, scoria Concepts to Review: Sedimentary Rocks New Vocabulary Weathering Outcrop Exfoliation Frost wedging Talus slope Hydrolysis Oxidation Soil horizons Clast Clay Boulder Cobble Pebble Sand Silt Bedload Lithification Cementation BIFs Evaporites Sedimentary environments Original horizontality Superposition Inclusions Faunal succession Bedding Cross bedding Graded bedding Ripples Mudcracks Formation Group Member Questions to answer: 1. Describe the formation of a clastic sedimentary rock from a preexisting igneous rock. Be sure to include each of the following steps: • • • • • Weathering Erosion Transport Deposition Lithification 3. What are the two most common types of chemical weathering? 4. How does the formation of a chemical sedimentary rock differ from the formation of a clastic sedimentary rock? 5. For each of the following sedimentary structures, indicate in what depositional environment they are usually formed: • Ripple marks • Cross beds • Graded beds • Mud cracks • Laminations 6. In terms of plate tectonics, in what locations are sedimentary rocks most commonly formed? Rocks to be familiar with: mudrock, claystone, shale, sandstone, conglomerate, breccia, coal, limestone, chert, travertine, dolostone Concepts to Review Metamorphic Rocks and Processes New Vocabulary Parent rock Foliated granoblastic Metamorphic grade Contact metamorphism Regional metamorphism Aureole Metasomatism Hydrothermal alteration Index mineral Questions to Answer 1. Why are metamorphic rocks formed at convergent boundaries more likely to be foliated? 2. How do index minerals help geologists determine the formation conditions of a metamorphic rock? 3. For each of the following types of metamorphism describe a tectonic setting in which it occurs: Regional metamorphism Contact metamorphism Hydrothermal metamorphism Rocks to know: slate, phyllite, schist, gneiss, migmatite, marble, quartzite, hornfels, greenstone/greenschist, blueschist, eclogite, skarn The Rock Cycle and Plate Tectonics – Study Group 2 From GEOL101 10L 11016 In this discussion you are participating with the whole class. Your first post is Due June 6 by midnight. The discussion closes June 9 at midnight. Please view the image below (adapted from Lecture Tutorials in Introductory Geoscience 3rd edition by Kortz and Smay 2020). The diagram below shows a cross section of the Earth’s tectonic plates. The ocean surface is indicated by a dashed line. А в с D E F The two box arrows point to the two plate boundaries illustrated here: the left arrow points to a convergent boundary with subduction and the right arrow points to a divergent boundary in the ocean. • For each letter (A-F) on the diagram, indicate what rock(s) might be FORMED there. There may be more than one correct answer for some locations. Explain why you chose that rock for that location, using your knowledge of geologic processes to support your reasoning. • Are there any locations on the map not marked by a letter that could form rocks? Where? Which rocks might form there. • Post your answers and discuss with your classmates any differences in your conclusions Discussion: Seattle geologic history – Study Group 2 From GEOL101 10L 1106 Post first before you can view your classmates’ posts. For full credit: 1. List three of the locations /features described in the article with which you are familiar. 2. Explain, in the context of the reading (Geology Too HighTooSteep.pdf ) how these places are related to the geologic history of Seattle. 3. Then comment on and ask questions about what at least 2 of your classmates have shared. FIRST POST DUE Sunday MAY 30 As an example: I have hiked up the long stairways on Queen Anne Hill, which is a drumlin shaped by the Puget Lobe of the Cordilleran ice sheet. I’ve been swimming in Lake Washington, whose very deep depths were carved by outwash streams beneath the retreating ice sheet. I live near a small round lake north of the city (Echo Lake) that is a perfect example of a kettle lake, created when large blocks of ice from the melting ice sheet are dropped on the landscape and left to melt.