Came across some old photographs I have of a classic Washington State geology site - Seattle's Magnolia bluffs AERIAL VIEW. The bluffs are located on the west side of of the Magnolia neighborhood. A large public park that was formerly part of Fort Lawton provides a wild landscape within the city.
The uppermost part of the bluff is nearly all sand. At the time of the pictures the upper bluff slope was a wind eroded landscape. The base of the bluff is a series of finely bedded alternating layers of clays and silts with some fine sand layers.
Upper Magnolia Bluff
Base of Magnolia Bluffs
Close up of base of bluff sediments
The upper sand unit is locally referred to as the Esperance Sand and the silt clay unit at the base of the bluff is locally called the Lawton Clay. These two units are a common sequence in the central Puget Sound bluffs and the unit terms are commonly used elsewhere as they are relatively recognizable units.
When glacial ice advanced out of the north into Puget Sound approximately 18,000 years ago the ice blocked the outlet of water to the north forming a deep glacial margin lake in what is now central Puget Sound. Initially glacial meltwater and streams flowing into the lake laid down the silts and clays of the Lawton Clay in alternating layers associated with the ebb and flow of meltwater flowing into the lake. As the ice advanced meltwater streams from the advancing ice dumped huge loads of sediment into the lake and as the ice got closer the lake became filled with sediment and sand became the dominant sediment forming the Esperance Sand.
Both the sand unit and underlying clay were over ridden by glacial ice and were highly compacted. The Lawton Clay was highly compressed and now stands as steep somewhat erosion resistant bluff slopes. The lack of clay and silt in overlying Esperance Sand means that sand has little cohesion. Hence, even though the Esperance sand is very compact, it readily erodes and looses stability when wet.
So at Magnolia Bluffs we have a great recipe for landslides: very compact, very low permeability Lawton Clay with low cohesion Esperance Sand sitting on top of it. It does rain in Seattle. The rain readily infiltrates into the and down through the sand until it reaches the clay. At that point the water can not go down and becomes perched on top of the clay and flows along the top of the clay. At Magnolia Bluffs the top of the clay slopes towards the bluff slope.
If the volume of perched groundwater builds up enough the sand bluffs become like sand castles at the beach getting wet - they collapse. A hike or scramble down the bluff slopes lead to springs and flowing streams from the perched water above the Lawton Clay and within the Esperance Sand.
Stream flowing across top of Lawton Clay with sand slope above
Jack strawed forest on a slide area
At the time of my venture on the slope I traversed a bizarre landscape of hummocky ground with trees pointed in all sorts of directions.
Large madrone tree hanging by roots over the edge of the bluff
Esperance Sand at the headwall scarp of slide complex to the right
The pictures shown above are from the park area. But down the coast is the now infamous Perkins Lane where homes built on the bluff were lost after a very wet weather event in 2007.
Perkins Lane on landslide area 1990
Perkins Lane area 2002