During his time teaching in Seattle, J Harlen Bretz noted the extensive array of glacial features present in the Puget Sound area. The Puget Lowland is an extensive glacial outwash plain of highly elongated drumlin topography adorned with kames, eskers, kettle lakes, glacial erratics, and other glacial features indicative of continental glaciation. The Seattle Basin is filled with a layer cake of distinctive sedimentary layers that reveal the advance and retreat of the Cordilleran Ice Sheet during the most recent (Wisconsin) glacial advance. South Beach at Discovery Park in Seattle offers visitors an easily accessible exposure of the stratigraphic layers which tell this story.
Magnolia Bluff at Discovery Park ranks as one of the most interesting and important geologic sites in the central Puget Sound Lowland. In its near-vertical cliffs, there is a record of the advance and retreat of the last great Ice Age glacier to enter western Washington from the extensive ice fields that covered southwestern Canada. At no other place near Seattle are the geologic relationships of this record so clearly displayed and so accessible to study. The exposure of sediment at Discovery Park is unique in that several distinct layers can be seen in the bluffs during a short walk along the beach.
To the inexperienced, the cliff appears to merely consist of a large pile of sand and clay; however, the different layers each tell a story of strongly contrasting environmental conditions over the past 25,000 years. Each of the major beds represents a depositional episode. Their depositional environments can be deciphered by close examination of the composition and texture of each layer of sediment.
Sediments comparable to those exposed at Discovery Park are found across Puget Sound, along Hood Canal to the south, and near Tacoma. The large regional extent of these deposits indicates that conditions throughout the central and southern Puget Sound Lowland were similar throughout the time period represented by the deposition of these sediments.
A half century of field investigations in the southern Puget Lowland (Armstrong et al., 1965; Crandell et al., 1958; Mullineaux et al., 1965; Noble & Wallace, 1966; Waldron et al., 1962) and in the northern Puget Lowland (Clague, 1981; Easterbrook, 1986, 1994; Troost & Booth, 2008) show that ice sheets have advanced south into the lowlands of western Washington.
Evidence for the latest glacial advance can be seen by examining the layer cake of sediments exposed at Discovery Park as the Cordilleran Ice Sheet advanced and retreated over the Seattle area. This recent advance is called the Vashon Stade of the Fraser glaciation.
Timeline
Prior to 100,000 years ago, the climate in Seattle must have been similar to the climate today. Evidence shows that a river system drained north through the Puget trough.
~100,000 years ago, Earth’s climate began to cool and became more moist. The Cordilleran Ice Sheet began to form in SE Alaska and in British Columbia.
~19,000 years ago, the Cordilleran Ice Sheet advanced to the Canada/US border, sliding at a rate of 135 meters/year. The ice sheet split into two lobes as it passed Victoria B.C. The Juan de Fuca Lobe extended out the Strait of Juan de Fuca, the Puget Lobe extended south over the Puget Sound region. When the ice reached Port Townsend, it blocked the north-flowing river that occupied the Puget Trough, forming a proglacial lake.
~18,000 years ago, ice overrode the lake and covered Seattle. Water was forced to drain to the south through the Chehalis river system.
~16,900 years ago, glacial maximum. The ice sheet extended to just south of Olympia, with ice thickness of ~3,000 feet over Seattle and 6,000 feet over Port Townsend.
Sediments visible at Discovery Park
Olympia Formation. This is the oldest formation on the exposed bluff wall. The Olympia Fm contains layers of sand, clay, and silt. These beds indicate a fluvial (river) non–glacial environment which existed in the Puget Sound region before the Vashon Stade glacial advance. The sand layers suggest stream deposition, the clay layers suggest ponding, and the silt layers suggest backwaters. There are shallow, broad ripple marks in some of the layers. Radiocarbon dates for wood in the layers at the base of the fm range from 22,000 to 20,000 years BP. A yellowish-gray silt layer 8 m above the bottom of the fm; this 2-m-thick layer contains wood fragments that are 18,000 years old. Pollen in the layers between the base of the Olympia Formation and the base of the overlying Lawton Formation are dominantly spruce and pine, representing a cooler climate than at present. These deposits reflect the climatic conditions in the area before the most recent arrival of ice in Puget Sound.
Lawton Formation. The Lawton Fm includes relatively dark clays above the brownish-gray non-glacial sediments. The clays represent deposition in the bottom of a lake which formed in ice margin lakes. The Lawton has striking laminations, which look like varves. The lower portion, which is older, is very fine-grained, which suggests that it was deposited in deep, calm water. The upper part grades into coarser materials and contains ripple marks. The oldest beds of the Lawton Fm are approx, 18,000 years old.
Esperance Formation. As the ice front of the Cordilleran Ice Sheet approached Seattle, the size of the deposited particles increased, grading from the clays of the Lawton Formation into the sand layer that we call the Esperance Fm. There are several different layers in this sand deposit, some of which are inclined. The sands are finer at the bottom and coarser on the top. The sands are well sorted, suggesting deposition from glacial meltwater, rather than by ice. The Esperance also contains lenses of gravel within the sand layers. Such lenticular bedding suggests a high energy environment of deposition. Geologically, layers or horizons in a deposit are referred to as “beds”. The orientation of the beds provides clues about the environment when these beds were deposited. The beds in the Esperance Fm show cross bedding. Cross bedding reveals flow direction.
Vashon Formation. (This formation is not visible in the image above, but can be seen nearby on the top of the cliffs.) The Vashon Fm is the glacial till, or ground moraine, deposited as the Cordilleran Ice Sheet retreated at the end of the most recent glaciation. It is a mix of clay, silt, sand, pebbles and boulders, which were carried along and ultimately deposited as the ice melted. Generally till deposits do not show any sort of bedding or size distribution. We know that during the most recent advance, ice overran the Seattle area about 18,000 years ago and retreated 16,800 years ago.
The Puget Lobe of the Ice Age Floods Institute hosts field trips to Discovery Park and to other glacial geology sites to introduce people to the fascinating glacial topography of western Washington. Additional field sites can be found in the Puget Lobe IAFI brochure, Our Cataclysmic Glacialscape.
Article by Dale Lehman, President of Puget Lobe Chapter