The Geology of Queen Anne Hill

Posted on by Marga Rose Hancock
Published 1993 by Queen Anne Historical Society, edited by Kay Reinartz, PhD
Chapter One:  The Geology of Queen Anne Hill

by Bill Laprade

The Puget Sound basin lies between the Cascade Range on the east and the Olympic Mountains on the west, and is open to the Pacific Ocean through the Strait of Juan de Fuca.  Bedrock ranging from 10 to 50 million years old in exposed at the ground surface around the margins of the basin and occasionally south of Alki Point and Boeing Field in Seattle.  Geologists estimate that the bedrock lies more than 1,500 to 2,400 feet beneath Queen Anne Hill, buried by glacial and non-glacial sediment in the past two or three million years.
The great ice ages commenced over three million years ago.  Geologic evidence indicates that at least four and perhaps as many as six glaciations have occurred since the beginning of the Pleistocene Epoch.  In the Puget Sound basin, ice originating in the coastal and inland mountains of British Columbia coalesced and progressed south, stopping approximately 50 miles south of Seattle.  Since the retreat of the last glacial ice from the Seattle area about 13,500 years ago, the land has been modified by rising sea levels, erosion, and landslides,  The waters of Puget Sound reached their present level about 5,000 years ago.
Egg-shaped Queen Anne Hill is one of the original seven hills of Seattle.  The others were Capitol, First, Beacon, Magnolia, West Seattle, and Denny.  The latter hill was removed in the Denny Regrade project.  Queen Anne Hill lies between three bodies of water — Puget Sound, Lake Union, and the Lake Washington Ship Canal — and the central business district of Seattle.  The deep depressions of Elliott Bay, Lake Union, and Lake Washington are filled with water and the uplands are mostly covered with human developments.

VASHON STADE ICE COVERS THE HILL
The geological materials exposed on or around the margins of Queen Anne Hill were deposited during the Vashon stade of the Fraser glaciations, the last incursion of continental ice into the central Puget Sound area.  A stade is a substage of a glacial period.  Of the three stades in the Fraser glaciation, only the Vashon deposited sediment in the greater Seattle area.  At its height, Vashon stade ice advanced past Olympia and covered Queen Anne Hill with more than 3,000 feet of ice.
Glacial remains of the Vashon stade on Queen Anne Hill are represented by four recognizable types:  Lawson Clay, Esperance Sand, Vashon till, and Vashon recessional deposits.  Lawton Clay is the oldest deposit and the others are progressively more recent.  All of these soils were deposited between 17,500 and 13,500 years ago.  Only the recessional deposits were not overridden by glacial ice.  A subsurface profile through the hill from north to south shows the relationship of the units to each other.  The interior of the hill is undoubtedly composed of Olympic non-glacial deposits from the interglacial period immediately before the Vashon Stade and of sediments from older generations.  These deposits and sediments, however, are not exposed at the ground surface or in shallow drill holes.

CLAY, SAND AND TILL

The oldest unit, Lawton Clay, is present below elevation 200 feet on the east, north, and west peripheries of the hill.  On the south slope, this material is covered with Vashon till.  Lawton Clay can also be seen in building excavations on the north side of the hill and on the steep excavation slopes along Westlake Avenue.  It represents the deposition of sediments in a lake that formed as the glacial ice advanced south and blocked the northern part of Puget Sound, eliminating the salt-water connection through the Strait of Juan de Fuca.  The clay consists of laminated to massive gray silt and clay with scattered thin layers of fine sand.  At Fort Lawton (Discovery Park), from which it takes its name and where it can be seen along the south beach, it is 82 feet thick; on Queen Anne Hill it ranges from 125 to 175 feet thick, as measured above sea level.
Overlying the Lawton Clay is the most extensively exposed geological unit on the hill, the Esperance Sand.  It is found on the steep slopes and on much of the top of the hill.  This deposit of sand, or mixture and sand and gravel, was laid down by streams issuing from the front of the advancing Vashon-Age ice sheet and for this reason is commonly termed advance outwash.  It is currently thought that the entire width of central Puget Sound basin was filled to elevation 400 to 500 feet with this deposit before the ice overrode the area.  On Queen Anne Hill, Esperance  Sand is about 150 to 200 feet thick.  It is found in the excavated slope to the northeast of the corner of Queen Anne Avenue North and Garfield Street, and it was well exposed during the excavation for the Queen Anne Pool at 1st Avenue West and West Howe Street.  Perhaps the best exposure of this deposit is along the top of the south bluff at Discovery Park.  The contact of the sand with the underlying Lawton Clay is commonly gradual and contains alternating layers of clay, silt, and sand.
As indicated on the subsurface profile, Vashon till overlies the advance outwash.  Vashon till, composed of clay, silt, sand, gravels, cobbles, and sporadic boulders, is commonly termed “hardpan” because of its very dense and compact nature.  This deposit is the debris that was carried along the base of the glacial ice.  It is not always present, but where it is, it ranges from a few feet to more than thirty feet thick.  It is limited in exposure on the top of the hill, but it can be seen in building excavations in the lower south side of Queen Anne.  One good viewing site is a parking lot excavation slope to the northeast of the intersection of Second Avenue W. and John Street, and it is also visible near the top of an old borrow pit just west of Twelfth Avenue W. and W. Howe Street.
The youngest geological unit on Queen Anne Hill associated with glaciation is Vashon recessional outwash.  It was probably laid down as streams flowed through the low area between Queen Anne Hill and Fremont when the glacial ice melted.  Found along the south side of the Lake Washington Ship Canal, this outwash deposit consists mainly of medium dense, silty sand, with minor amounts of gravel.  It is also deeply buried below fill in the Interbay lowland, as that area served as an outwash channel during the recession of the glacial ice.  Recessional outwash sand is also commonly found on the uplands in isolated pockets no more than a few feet thick, overlying till.

HOLOCENE EVENTS
Following the recession and wasting of glacial ice in central Puget Sound about 13,500 years ago, geologic processes began that continue to this day:  erosion and gully formation, lowland filling, hillside soil weathering, and landsliding.  The rate at which these processes initially occurred is unknown, but it is likely that all of them were more rapid on the denuded, recently deglaciated slopes.  For the past several thousand years, the rate of erosion, sedimentation and land-sliding has probably been somewhat constant.  During the same time period, sea level rose gradually about 300 feet to its present level.
When European American settlers arrived in Seattle in the mid 1800s, Queen Anne’s sister hill, Magnolia, was virtually cut off from the mainland by the nearly-connecting tide flats of Salmon Bay to the north and Smith Cove to the south.  The tide flats were covered with soft mud and sand, and peat deposits dotted the surface.  The edges of Interbay were mantled with landslide deposits that came from the hillsides to the east and west.
Minor gullies have developed on the post-glacial landscape throughout Queen Anne Hill, but only one has developed into a deep ravine.  The half-mile-long ravine on the northeast side of Queen Anne Hill has its mouth near the toe of the north slope, and currently extends to Lynn Street on the south, following the right-of-way of Third Avenue North.  A 1910 Seattle Engineering Department topographic map shows that this gully once extended much farther south, to the vicinity of Newton Street, but it was subsequently filled in.  Prior to the placement of the fill over which the present roadway is built, a 300-foot-long bridge spanned the creek along Boston Street over a 55-foot-deep ravine.
Three geologic factors appear to be responsible for the formation of this gully:  (1) erosion by the ubiquitous springs in the vicinity of Queen Anne North Drive, (2) the impervious till cap on the upland to the south of the gully, which promoted surface runoff of precipitation, and (3) the highly erosive Esperance Sand in the area between the springs and the drainage basin to the south.  Two high structures, the McGraw Street Bridge and North Queen Anne Drive Bridge, were built across the ravine by the Seattle Engineering Department in 1936, with General Construction as the contractor.
The slopes around the edge of Queen Anne Hill are very steep in many places.  The soil on these slopes is loosened by gravity, root action, freeze-thaw cycles, and chemical changes.  Throughout the Puget Sound area, this loosened soil rind, termed colluvium, is commonly three to ten feet thick.  Based on drill holes for residential developments, colluvium is as thick as 15 feet on the east slope, west of Aurora Avenue.  The bowing and bending of the trees that grow on the hillside are a result of the downslope movement of the colluvial soil.  This rind of colluvium is commonly involved in landslides, as it is loose and susceptible to the absorption of water in its inter-particles’ void spaces.
Landslides were and continue to be an important geological process in the sculpting of the hill’s topography, especially on the east and west sides of the hill.  Many of the landforms on these flanks of the hill still exhibit classic landslide topography, such as hummocky ground at the bottoms of steep slopes and steep-sided amphitheaters or bowls.

THE SPRINGS
Springs are common to many of the landslide-prone areas on Queen Anne Hill, and are located around all sides of the hill.  They normally run throughout the year but are especially productive in the winter.  On the east side of the hill, the spring line is between elevations 150 and 200 feet.  On the north side, it is found between elevations 100 and 150 feet, and on the west side it is observed between elevations 125 and 200 feet.  The most prolific springs on Queen Anne Hill existed on the south slope in the vicinity of Fourth Avenue North and Ward Street  before they were incorporated into the city storm drainage system.  Nearly all of the springs are found at the contact between the Lawton Clay and the overlying Esperance Sand.  Precipitation infiltrates into the ground and travels vertically through the pervious sand until it encounters the top of the more impervious clay and silt of the Lawton Clay.  The water then moves laterally until it emerges from the slope in springs.  Many of the springs around the periphery of Queen Anne Hill were used for drinking water by early settlers.
Between 1882 and 1890, five spring-fed water supply systems known as Union, Maggs, Griffith, Kinnear, and Peterson were installed on Queen Anne Hill.  The Union Water System was one of the largest water supply systems in the city, yielding about 80,000 gallons per day.  It was located at the large spring in the vicinity of Fourth Avenue North and Ward Street.  The Maggs System, in the vicinity of Seventh Avenue North and Garfield Street, provided water to the southeast portion of the hill and portions of the Denny Regrade area as recently as 1950.  Springs are still found scattered around the hill, but most have been captured by drainage trenches or tunnels and have been routed into the storm drainage system.
 
LANDSLIDES
Landsliding is closely related to the presence of springs due to the internal pressures that build up in the ground when water is blocked or inhibited from escaping.  The zone of particular hazard for landslides on Queen Anne Hill is defined by a band around the hill on the east, north, and west sides.  It is commonly referred to as “The Contact” because it is defined by the contact between the Lawton Clay and Esperance Sand.  Larger slides appear to be related to a gradual increase in the water table over the winter months followed by an intense or prolonged period of rain.  These slides are commonly slumps that form large amphitheaters.  Such topography is common on the east side of Queen Anne Hill, uphill from Aurora Avenue, and between Howe and Galer Streets, as well as along the west side of the hill, east of Elliott Avenue West and Fifteenth Avenue West, and between West Howe and West Comstock streets.
The West Galer Street landslides that occurred in 1909, 1916, and from 1951 to 1954 are examples of a slump.  Much of the material that slid off the bluff just south of the ramp to the Magnolia Bridge came to rest behind a restaurant at West Garfield Street and Fifteenth Avenue West and was hauled away by Seattle Engineering Department crews during the 1950s episode.  Horizontally drilled drains to relieve potentially destabilizing pressure deep in the ground were installed in 1988 during construction of residential units adjacent to the slide area.

On December 11, 1983, a large mass of saturated earth flowed down the east side of Queen Anne Hill, crossed Aurora Avenue and stopped on Dexter Avenue.  Although this mudflow occurred along “The Contact,” and natural spring seepage was a contributing factor, buried drain lines and fill placed on the hillside over the top of the spring water sources were also causes.

THE DENNY REGRADE

Above:  Denny Regrade No. 1 in 1910.  Pinnacles of land remained where hold-out owners refused to sell.  Those owners later were required to excavate at their own expense.  Excavation after 1906 was made by hydraulic sluicing.  Courtesy University of Washington, Asahel Curtis Collection

One of the most remarkable civil works projects in the Pacific Northwest was the removal of Denny Hill, just south of Queen Anne Hill, between 1903 and 1926.  This area is now called the Denny Regrade.  In a scheme envisioned and championed by R. H. Thomson, the Seattle Engineer in the early 1900s, several parts of the city were regraded to open it up and improve the transportation network.
The regrading of Denny Hill was the first of these projects undertaken by the city and was accomplished in two phases.  The first regrade, on what is now the western half of the area, was performed between 1903 and 1911 and used hydraulic sluicing methods to wash the soil into Elliott Bay.  In 1928 the eastern half of the area was levelled using electrically-powered shovels and a complex series of conveyors.  These two regrades accounted for the removal of six million cubic yards of soil from an area of 62 city blocks.

LANDSCAPE CHANGES ON QUEEN ANNE HILL AND SURROUNDINGS
Lake Union was formerly connected to Salmon Bay by a small creek that was enlarged to a narrow canal for log transit.  As part of an ambitious scheme to change the drainage of Lake Washington, between 1911 and 1916 the Sammamish River, the Cedar River, Lake Washington, Union Bay, and Lake Union were directed through the newly-engineered Lake Washington Ship Canal.  The Fremont Cut was part of this project.  The excavation was made into Holocene-age alluvium and colluvium, essentially soft mud.  Harder Vashon till and Lawton Clay were encountered in the bottom of much of the excavation.  The excavation of mud, sand, and soft clay was accomplished mostly by hydraulic sluicing and steam shovel.  Much of the material was pumped into adjacent low-lying ground.  Concrete walls along the sides of the canal, extending several feet below water level for wave protection, are supported on piles.

The earliest transformation of Interbay was performed around 1910.  This involved dredging a channel to lower the water table, thereby drying other areas for athletic fields.  A limited amount of filling was also done at that time.  Filling, chiefly as an open-air landfill dump, continued intermittently from 1911 to 1968.  The thickness of the fill has reportedly settled a great deal and the presence of methane and other gases in the landfill has been well documented.
The construction of Aurora Avenue was completed along the alignment of Seventh Avenue North in 1932 as part of the Pacific Coast Highway chain.  It significantly impacted the east side of Queen Anne Hill, cutting through the landslide zones there and triggering slope instability in several places.  Landslides were curtailed after stabilizing measures were undertaken in 1933 by the Seattle Engineering Department.  However, an occasional slide still occurs.
With regard to landscape changes, the two most geologically significant periods were probably the first few hundred years after the disappearance of the Vashon stade ice and the last one hundred years when European American settlers moved in.  Earlier modifications were large, with concomitant environmental consequences; smaller changes will be the watchword of the future.  Limitations spelled out by Seattle’s Sensitive Areas Ordinance and the State of Washington’s Growth Management Act will enable us to move into the twenty-first century in a sensible manner.

Bill Laprade has lived on Queen Anne Hill since 1974.  His spouse, Mary Lou, teaches at John Hay School and two sons, Jed and Joseph, have been active in sports and community affairs on the hill.  Bill coached soccer for 11 years in the Queen  Anne Soccer Club.  He is an Associate with Shannon & Wilson, Inc., a geotechnical engineering and environmental consulting firm, where he has practiced engineering geology for twenty years.

Reference: QUEEN ANNE GEOLOGIC HISTORY with David B. Williams