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The Hill We Call Magnolia: Puget Lowland Geology

By Bill Laprade

The hill we call Magnolia sits in the middle of a broad basin between two towering mountain ranges. The Olympics, to the west, are still climbing skyward due to the inexorable northeastward push of the Juan de Fuca tectonic plate into the North American tectonic plate. The Cascade Range, to the east, is formed by old and new magma from the depths of the diving Juan de Fuca plate, and by ancient plates that attached themselves to the North American continent (1).

 

The broad Puget Lowland basin is composed of younger bedrock, sometimes at the surface and sometimes buried, but widely covered by glacial deposits from at least eight incursions of ice. The land within this basin was carved by glacial waters into deep ravines and rolling hills. Many of the valleys and ravines are now drowned by saltwater and freshwater. The gently rolling hills, one of which is Magnolia, are now populated by thousands of people.

 

The geology of Magnolia tells a story that will help you understand the ground on which you tread in your everyday comings and goings throughout the neighborhood. The steep climb up West Dravus Street, the magnificent cliffs of Discovery Park, the flat BNSF train yard, and the level ground on West McGraw Street in the Village inspire us to ponder the geologic reasons for all that we see.

Topography of our hill

Magnolia comprises two long, north-oriented hills separated by a valley known as the Village. The long ridges are about the same elevation (west at 393 feet and east at 369 feet), and incline gently down from north to south (2, 3). On the east, the moderate slope angles down to Interbay. On the west and south, the slopes end abruptly at the high cliffs of Magnolia Bluff, on the shoreline of Puget Sound. Wolf Creek, although heavily human-modified, splits the ridges at the south end. To the north, gradual slopes end at the Lake Washington Ship Canal, and Kiwanis Ravine empties out near the Chittenden Locks.

Fig. 1. A Light Detection and Ranging (LiDAR) image of Magnolia, created from airborne pulsed lasers, highlights natural and human-modified topographic features.

Courtesy of Shannon & Wilson, Inc. 2025.

The Quaternary Period in Magnolia

Magnolia is made up of sediments deposited and shaped during the Quaternary Period, which spans the last two-and-a-half million years (4). The Quaternary is divided into two epochs: the Pleistocene (2.5 million to about 11,000 years before the present), commonly known as the Ice Ages, and the Holocene, or post-glacial time.

 

Pleistocene Epoch

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Glacial movements during the Pleistocene Epoch are evidenced by the geologic layers that make up the Puget Lowland—from the deepest layer, the Olympia Beds, to the recessional outwash.

 

Olympia Beds: We know that the oldest exposed sediments in Magnolia are about 20,000 years old (5). The nonglacial and layered brown sand, silt, and clay found near the base of the South Bluff at Discovery Park date from 18,000 to 22,000 years old, the product of streams and lakes from the last interglacial period. This formation is exposed elsewhere in Seattle, including Alki in West Seattle, but it takes its name, Olympia Beds, from an exposure in Olympia, Washington.

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The Olympia Beds can be seen at South Beach, where they form a vertical cliff. They are inclined down to the south in such a way that they disappear below the beach at the southern border of Discovery Park. Farther north, where they are most exposed, vertical cracks are evident. Those cracks have been exploited by waves to form caves at the beach level. Periodically, large chunks of the formation calve off the face, landing on the beach. Over the winter, storm waves wash that material northward toward the lighthouse.

Fig. 2. South Beach Bluff photo taken at low tide showing the three main geologic components of the bluff (from bottom to top):

Olympia Beds, Lawton Clay, and Advance Outwash.

Photo by Marcus Donner. 2024

Lawton Clay: As glacial ice built up to considerable thickness in British Columbia, it slowly advanced south, about 20,000 years ago, and blocked the Strait of Juan de Fuca. The water in ancient Puget Sound no longer had a direct outlet to the Pacific Ocean, so a large inland lake was established in the Lowland, fed by the rivers coming from the Cascades and Olympics and, most voluminously, from the glacial ice itself. The water eventually found an outlet to the south, emptying out along the Chehalis River and into the sea near Aberdeen, Washington.

 

In the glacial lake, the fine sediment generated by the grinding action of the ice slowly settled to the bottom, building up a thick sequence of clay and silt with some thin layers of fine sand. This is the gray layer that can be seen just above the Olympia Beds at South Beach. It can also be walked upon on the beach near Discovery Park’s southern border or on its northern end during low tide. The Lawton Clay layer is about 100 feet thick at the South Bluff.

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Lawton Clay takes its name from when the park was named Fort Lawton. Geologists call this the “type locality”; it is a reference to the first place it was measured and formally documented (6).

Fig. 3. Exposure of gray Lawton Clay just above the Olympia Beds on the South Beach Bluff.

Photo by Marcus Donner, 2024

Advance outwash/Esperance Sand: As the ice advanced southward toward its eventual destination near present-day Olympia, Washington, the energy in the streams flowing from the front of the ice was able to move larger sediment, such as sand and gravel. This is the light brown sand ubiquitous in the South Bluff, where it stands as high as 70 to 100 feet. It can be seen in a small alcove near the sand dune at the top of the South Bluff.

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Sand accumulated as deep as 150 feet here in Magnolia. Although Discovery Park has one of the best exposures of this formation in the Puget Lowland, it takes its name, Esperance, from a defunct sand and gravel borrow pit near Edmonds, Washington. More recently, geologists refer to it by its generic name, advance outwash, and its type locality is controversially listed as the South Bluff.

 

The outwash on Magnolia is almost entirely sand and contains very few fine soil particles, such as clay and silt, so it is quite permeable. As a result, most of the rainfall infiltrates directly into the ground. The water percolates down to the Lawton Clay layer, which has very low permeability, then perches on top of it and flows along its buried topography toward Puget Sound, popping out as springs in low spots and on slopes around the hill (7).

Fig. 4. Advance outwash exposure in small alcove at the top of the South Bluff, adjacent to the Discovery Park Loop Trail and sand dune area. Note cross-bedding of the sand layers.

Photo by Bill Laprade. 2024

Vashon Till: A layer of soil developed between the base of the glacial ice and the ground surface, consisting of many varied particle sizes from clay to boulders. It is not sorted by water, so all the soil particles are mixed up. Vashon Till is commonly referred to as “hardpan” in the Puget Sound area and has the appearance of concrete. Its type locality is the shoreline cliffs of Vashon Island, but it is found throughout the Puget Lowland, particularly on ridgetops. On Magnolia, it is exposed just above the beach kiosk at South Beach, but it can also be encountered in shallow shovel holes on both ridges at the southern half of Magnolia.

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Till is dense, gray, silty, gravelly sand with scattered cobbles and boulders. It is very difficult to excavate and is practically impervious to water (8). The boulders on the beach along Magnolia Bluff were once part of the till, before the finer matrix and underlying sediment were washed away by the waters of Puget Sound.

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Vashon Till comprises the surface soil and outcrops on the sea bluffs at the northern end of Magnolia, in Discovery Park and Lawtonwood (the neighborhood just north of the park). On the beach at Discovery Park, it can be seen on the low cliff just above the beach kiosk. It is present all along a narrow band of the eastern side of the hill but is not well exposed due to vegetation and residential development. At the southern end of Magnolia, Vashon Till is found at the ground surface on both ridges, including the southern end of Perkins Lane, extending eastward to 32nd Avenue West.

Fig. 5. Vashon Till exposure just above the South Beach interpretive kiosk. Note the concrete-looking texture and the gravel and pebbles in the matrix.

Photo by Bill Laprade. 2025.

Recessional outwash: As the ice wasted and receded back from the Seattle area, high-energy streams once again flowed from the glacial front, laying down coarse-grained particles such as sand and gravel. This material filled the north-oriented valleys and swales—it partially filled Interbay to the east of the hill and the Village swale between the ridges.

 

The brown recessional outwash is loose because it was not compressed by the weight of the ice sheet, unlike the older dense deposits beneath. It mostly consists of fine to medium sand with a little silt in the matrix in the Magnolia area. It is permeable and easy to excavate (9).

 

Advance outwash covers most of Magnolia’s inland surface (10).

Holocene Epoch

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Shoreline recession and deposition: When the Ice Age ice sheet disappeared from Magnolia, the shoreline was less than a mile west of its present position (11). Glacial ice retreated to the north, and the Strait of Juan de Fuca was once again open to the sea, leaving a deep, steep-sided canyon in what is now the middle of Puget Sound. When sea level once again rose due to worldwide glacial melting and reached its approximate present level about 4,000 years ago, the waves of Puget Sound started their inexorable erosion of the bluff (12). The periodic instability seen in the Magnolia Bluff today is typical of what has been occurring for the past 4,000 years. And it will continue (13).

 

The South Bluff, and a short section at the northern end of North Beach at Discovery Park, are the only remaining unprotected, undisturbed shoreline slopes in Seattle, other than short individual sections of private property. The rate of shoreline bluff erosion has been considerable at Discovery Park (14). Calculated retreat rates over the past 16,000 years at the North Beach bluff and South Beach bluff are about 1 inch and 2.6 inches per year respectively. The rate is four times faster if only the past 4,000 years are taken into account, when sea level was at its present level. This average is not, however, how the bluff actually retreats. The shoreline cliffs recede by episodes of a few feet to tens of feet. The rate of recession is not equal along different parts of the bluff. That depends on the amount of shoreline protection (walls and rock bulkheads, for example), the geologic formation present at beach level, the amount of groundwater seepage on the slope above the beach, and the presence of natural protective landforms such as the West Point spit.

 

One of the more significant Holocene geologic features in the Seattle area is the West Point spit. It is built primarily of sand eroded from the South Bluff feeder bluffs. The coarse material is delivered to the beach by landslides on the slopes above. The prevailing storm winds come from the south and southwest and move the beach material to the spit. The pointed shape of the spit is a result of periodic, strong winter north winds that come out of Canada approximately every decade and push sand from North Beach to the spit. The spit was occupied by Duwamish peoples for millennia. The northern side is now the site of the West Point Treatment Plant.

Landslides

Seattle is one of the most landslide-prone cities in the world, and Magnolia provides its ample share to Seattle’s total. Since records have been kept by the City of Seattle, more than 1,400 landslides are documented, according to documents gleaned from city archives and engineering consultants’ files (15). As can be seen from a LiDAR image, hummocky and steep topography is abundant around the edges of the hill, particularly on the slopes adjacent to Puget Sound.

Fig. 6. Magnolia area from Seattle Landslide Study by Shannon & Wilson, Inc. for Seattle Public Utilities, 2000. Note concentrations of landslides on the western side, particularly Perkins Lane, and the eastern side, along Thorndyke Avenue West.

Landslides have been mapped and categorized for Seattle, including Magnolia, five times since 1973 (16). As shown on the map from the Seattle Landslide Study (figure 6), Magnolia landslides are heavily concentrated along Perkins Lane to the west, the slopes near the southern end of 32nd Avenue West, the slopes west of Interbay (Thorndyke Avenue West), and along the Lake Washington Ship Canal. Discovery Park has a plethora of landslides, but they are not reported, nor have they been stabilized, because they are on Seattle Parks and Recreation property that is not actively managed.

 

Landslides on interior land, such as Interbay, 32nd Avenue West, or Kiwanis Ravine, cause a loss of ground at their heads and accumulation of debris (earth and trees) at the bottom of the slope. When landslides occur along the Puget Sound shoreline, the debris is washed away and deposited along the beach. As evident along Discovery Park’s South Beach, the sand and silt are winnowed out, leaving a gravelly surface on the beach as well as logs and trees that have come down with the landslide.

 

Of particular significance for Magnolia was the spate of landslides in the consecutive winters of 1995–96 and 1996–97. More than twenty landslides occurred along Perkins Lane, damaging and destroying residences and parts of the street. Several landslide repairs were constructed along the lane over the following few years to increase stability. Damaging landslides also occurred in and adjacent to the 32nd Avenue West ravine, and at the short section of West Galer Street at the southern end of 32nd Avenue West.

 

In the early 1980s, a large, deep-seated landslide occurred near the present location of the Elliott Bay Marina (17). The landslide threatened the residences close to the bluff’s edge, but more significantly, it severed the large sewer main buried in the beach that delivers wastewater from downtown Seattle to the wastewater treatment facility at West Point. This landslide also had major consequences for the construction of the marina. The large fill for the parking lots and buildings, as deep as twenty-five feet, served to buttress the hillside and stop the chronic bluff instability. Construction for the marina and supporting facilities started in 1989, after ten years of permitting, and was completed in 1991.

 

In January 1997, another critical piece of infrastructure was damaged by a landslide on the slope at the western end of the Magnolia Bridge. The head of the landslide endangered the residences above the bridge, and the debris at the toe slightly damaged bridge supports (18).

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Emergency action by the Seattle Department of Transportation, its consulting engineers, and on-call contractors stabilized the slope with a large reinforced-concrete retaining wall. The bridge was back in service by spring. The city bought the houses on the brink to allow for construction of the wall, then resold the properties when construction was complete.

Fig. 7. Head scarp of landslide at the western end of the Magnolia Bridge. Note the undermining of the residence above. A concrete retaining wall now covers this scar.

Courtesy of Shannon & Wilson, Inc. 1997.

Human modification

On much of the hilly topography of Seattle, the government and private developers modified, cut, and filled in land to spur or enhance ground for development. Although not on the grand scale of the Denny Regrades between the 1890s and 1920s, Magnolia has had some ground-changing events.

 

The earliest and longest-lasting project is the work at Fort Lawton, which started with the forest clearing and original grading in 1897 and continued sporadically through the World War II days. At the end of the nineteenth century, the road network was laid out, rifle range flattened, and hilly ground made level for parade grounds and structures. In the 1940s, additional ground was prepared for structures for the thousands who passed through on their way to the Pacific Theater. If you look at the LiDAR image of Discovery Park (figure 8), you see broad areas of leveled ground that were once forested, then modified, and are now once again vegetated.

Fig. 8. A LiDAR image of Discovery Park highlights the unstable slopes along the western side of the park

and the widespread ground modifications at Fort Lawton.

Courtesy of Shannon & Wilson, Inc. 2025.

The ground north of the Village was leveled for the playfields to a minor extent—for example, only two to three feet of fill at West Ray Street. However, in the heart of the Village it was a different story. Originally, the 32nd Avenue West ravine (once known as Wolf Creek) extended northward to West McGraw Street and well beyond, resulting in the filling of twelve to fifteen feet of fill in that drainage draw at West McGraw (19).

 

Interbay’s topography has also been dramatically altered since it was carved out by subglacial water flow during the last ice occupation. After the ice wasted and retreated north, depositing recessional outwash in the Interbay trough, the land remained in that condition with mudflats and tidal fluctuations for at least four millennia. The only dry ground was in the vicinity of West Dravus Street. It’s no wonder that Dr. Henry Smith bought property there and developed that area known as “Boulevard” in the 1850s (see figure 9). In 1884, the Seattle, Lake Shore and Eastern Railway bought ninety-five percent of Smith’s property (20).

Fig. 9. An 1894 topographic map of Seattle indicates the full extension of Smith Cove northward into mudflats, almost to the present West Dravus Street before it was filled between 1911 and 1916. From the vicinity of West Dravus, a creek runs northward to Salmon Bay.

Image source: US Geological Survey. 1894.

However, the open waters of Smith Cove and the Interbay mudflats to its north were filled extensively between 1911 and 1916. Interbay had been selected as a convenient location for depositing excavated soil when the US government finally decided on a route linking Puget Sound with Lake Washington.

Fig. 10. A 1903 photo looks southeast from Magnolia (Queen Anne in the background), showing the Smith Cove tideflats before filling commenced in 1911. The former Wheeler Street Bridge spans the tideflats.

Image source: Seattle Municipal Archives, No. 30031.

City of Seattle records indicate fill was placed as deep as thirty feet along the alignment of West Ray Street and forty-five feet along the alignment of West Garfield Street. Additional filling of Interbay occurred south of Dravus for an open-air landfill. After decades of operation, the Interbay dump was closed by the city in 1957, and now, after an earth cap was placed over the dump and methane wells were installed to vent the trapped gases, the area hosts the Interbay Golf Center and a neighborhood P-Patch.

 

One of the most fascinating geologic modification stories pertaining to Magnolia is well hidden beneath the ground. During the very wet winter of 1933–34, a large number of landslides devastated Seattle. Houses, roads, and commercial structures were damaged or destroyed throughout the city. The Seattle Engineering Department proposed twenty-nine projects for landslide mitigation citywide, ten of which were in Magnolia.

Fig. 11. This map shows the ten areas in Magnolia where landslide mitigation projects incorporating drainage trenches, tunnels, and shafts were constructed during the years 1935–1941.

Image source: Seattle Engineering Department. 1967.

Perkins Lane led the city with six landslide projects. A Seattle Engineering Department engineer reported: “It has been a problem to the City for slide troubles and maintenance ever since it was originally graded in 1926…” Four additional projects were chosen and completed on Thorndyke Avenue West (21). Citywide, the projects cost a million dollars, of which the federal Works Progress Administration (WPA) paid $850,000 (22).

 

The Magnolia projects consisted of drainage trenches, tunnels, and shafts. A typical project schedule included a preliminary geologic evaluation, test holes that extended below the landslide materials, analysis, engineering drawings, and then construction performed by hand labor. Pipes were placed at the bottom of the trenches at least two feet below the landslide zone. The trenches were typically thirty inches wide and filled with gravel. Tunnels were four feet wide and six feet high. The tunnels and shafts were interconnected with the trenches to capture and convey groundwater to Puget Sound (23).

“Geology reigns”

The ground we walk on, and view from afar with awe, is a result of millions of years of geologic processes. Magnolia provides an excellent example, particularly of the past 24,000 years. Whether you’re walking Magnolia Bluff, crossing the Dravus Street Bridge, or strolling through the Village, it’s interesting to think about how geology has shaped our world—and what we have done to change the original layout to our liking. In the geologic sense, the earth beneath our feet is dynamic, changing with storms, landslides, earthquakes, and groundwater flow. Geology forms the foundation of our world, from our building materials to the ground on which we live.

Bill Laprade is a retired engineering geologist, having worked with Shannon & Wilson, Inc., a local geotechnical and environmental consulting firm, for fifty years. A Queen Anne resident since 1974, he also penned the first chapter (“Geology of Queen Anne Hill”) in Queen Anne: Community on the Hill, 1993. Among his many other publications: coauthor of Geology of Seattle, Washington, USA and principal author of the Seattle Landslide Study. He teaches tai chi and serves on the boards of the Friends of Discovery Park and the Edmonds Pétanque Club.

Notes

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  1. E. A. Barnett et al. Preliminary Atlas of Active Shallow Tectonic Deformation in the Puget Lowland, Washington. US Geological Survey, 2010, https://doi.org/10.3133/ofr20101149.

  2. “East Magnolia, Washington,” 11 Jan. 2025, Peakbagger.com, https://www.peakbagger.com/peak.aspx?pid=-88069.

  3. “Magnolia Hill, Washington,” 11 Jan. 2025, Peakbagger.com, https://www.peakbagger.com/peak.aspx?pid=2034.

  4.  K. G. Troost and D. B. Booth. “Geology of Seattle and the Seattle Area, Washington,” Landslides and Engineering Geology of the Seattle, Washington Area: Reviews in Engineering Geology XX. Edited by R. L. Baum et al, The Geological Society of America, 2008, pp. 1–35.

  5. “Geology of Seattle and the Seattle Area.”

  6. D. R. Mullineaux, H. H. Waldron, and M. Rubin. “Stratigraphy and Chronology of Late Interglacial and Early Vashon Time in the Seattle Area, Washington,” U.S Geological Survey Bulletin 1194-O, 1965.

  7. R. W. Galster and W. T. Laprade. “Geology of Seattle, Washington, United States of America,” Bulletin: Association of Engineering Geologists vol. 28, 1991, p. 239–302.

  8. “Geology of Seattle, Washington.”

  9. “Geology of Seattle, Washington.”

  10. K. G. Troost et al. “The Geologic Map of Seattle, a Progress Report,” US Geological Survey Open-File Report 2005-1252.

  11. Nicole Sarriedine. 2021, “Establishing Baseline Monitoring for Retreat of the North and South Beach Bluffs of Discovery Park, Seattle, Washington,” MESSAGe Technical Report 095, University of Washington Earth and Space Sciences: Applied Geosciences, 2021. http://hdl.handle.net/1773/46897.

  12. “Geology of Seattle and the Seattle Area.”

  13. H. Shipman. “Coastal Bluffs and Sea Cliffs on Puget Sound, Washington,” US Geological Survey Professional Paper 1693, p. 81–91.

  14. “Establishing Baseline Monitoring.”

  15. Shannon & Wilson, Inc. “Seattle Landslide Study, Seattle, Washington, Prepared for Seattle Public Utilities,” 2000.

  16. W. T. Laprade and D. W. Tubbs. “Landslide Mapping in Seattle, Washington,” Landslides and Engineering Geology of the Seattle, Washington Area: Reviews in Engineering Geology XX. Edited by R. L. Baum et al, The Geological Society of America, 2008, pp. 37–54.

  17. GeoEngineers, Inc. “Final Geotechnical Report, Evaluation of Landslide Stability, Magnolia Interceptor Sewer, Seattle, Washington, for Municipality of Metropolitan Seattle,” 1984.

  18. Shannon & Wilson, Inc. “Geotechnical Report, Magnolia Bridge Slide Repair, for Sverdrup Civil, Inc. and Seattle Department of Transportation,” 1997.

  19. Seattle Digital Infrastructure Records, City of Seattle Records Vault.

  20. Monica Wooton. “The Interbay Dump,” Magnolia: Midcentury Memories, Magnolia Historical Society, 2020, pp. 41–43.

  21. Henry M. Fitch. “WPA Slide Control Drainage Projects (1935-1941),” Seattle Engineering Department, 1967.

  22. “WPA Slide Control.”

  23. “WPA Slide Control.”

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