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.
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.
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.
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.