March 9 2008 Hammond walk with Jim Johannesen · 10 March 2008, 09:24 by Julie Loyd
Scroll down to find three more related articles about Jim’s visit. Please note that underlined phrases are related links that you can click on. Some changes to this post were suggested by Jim, which were incorporated April 4.
We started at Mittelstadt’s with a geological map of the area.
The map shows that the bulk of the island is glacial till left by the Frazer Glacier, called Outwash gravel, Vashon Stade. Disney is composed of Cretaceous Nearshore sedimentary rocks, from the Nanaimo formation.
Jim answered questions:
Turbidite deposits come from submarine landslides, making a unique sequence of underwater rock: 1. Big rocks fall first, making a conglomerate layer. 2. sands catch up. 3. Layers of fine sediment precipitate out of the water column.
The Pacific Ocean Plate acts as a conveyor belt bringing material up from the south. Vancouver Island is Wrangellia terrane, as are parts of the Yukon and Vietnam – partly deduced from the presence of foraminifera. See John McPhee’s Assembling California.
As the conveyor belt goes along, islands are scraped off and glued on to the continental plate. These terranes go as far east as the Idaho panhandle. That was about 100 million years ago.
Every 260 – 1150 years, there’s a subduction zone earthquake as the ground under us shifts 30 – 40 feet. It has been about 300 years since such a quake, deduced from tree-ring and Japanese tsunami records.
The San Juans were scraped off in layers, so our geology is a mish-mash. The Nanaimo formation, found at Disney, is 90 million years old, and is very hard, deepwater sedimentary rocks, mostly composed of conglomerate and sandstone, also with basalt (ocean floor rock).
Now we ventured out to the beach, walking towards Hammond. What evidence do we see of littoral drift? In the standard model, you would see boulders and cobbles at one end of the beach, tapering off to a broader sandy beach at the other end where the wave energy was less. On this beach, either end has cobbles. This reflects that, whatever the littoral drift is doing, the forces that most strongly shape a beach are the storms. Our Northeasters come from a different direction than the prevailing winds.
Look at the bluffs. You can see various striation patterns. As the glacier advanced, streams would be running in front, perhaps frozen at night. Diagonal striations show where the stream bars advanced laterally. Where rounded (cup-shaped) deposits are present, that is called cut and fill structures as areas were eroded then filled in.
Does each different stripe represent a daily pulse due to ice melting and then freezing? Or some other time unit? Anyway, we know the darker bands have more organic material in them. The oldest sand would be at the bottom, deposited 30 – 20,000 years ago. Then, near the top of the bluffs, you see gravel. This material would have come from the glacier itself, instead of the streams running ahead of it. Gravel melts out as the glacier advances. Finally, at the very top you have glacial till, concrete-like material with occasional boulders.
On Whidbey Island, you can see evidence of yet two more glacial ice sheet advances beneath this one.
At Little Hammond, we talked about the two bars extending seaward. Underwater sand movement is not well understood, nor is its role in shorelines.
Little Hammond Bay shows a clear log spiral (see article below) where the southernmost bluff portion is deeply eroded.
The tombolo at Hammond has been getting smaller over the years. Don thought it was eroding from the bluff. Jim said it was mapped (by him in 1992 for WA Dept of Ecology) as the area where two net shore-drift cells converge, such that it was deposited by littoral drift. That does not preclude cyclical erosion, as much of the island’s shores are eroding.
We looked at the scallops in the clay. We looked at the scallops in the clay. The clay shows that the last of the glacial ice sheet was floated up by rising ocean levels at the end of the last glacial advance around 11,000 years ago. All of the sediment that was left in the ice was dropped in place forming “glaciomarine drift”, with the scallop shells buried simultaneously.
If you took pollen samples, you might be able to track the typical post-glacier pattern. First, arctic grasses appear, then alders, then pines, then firs, and finally, cedars. Our personal cedars appeared here about 5,000 years ago.
Geologically, what happens while the plants are reestablishing, is that the ice melts, then marine water floods in with its attendant flora and fauna. Eventually, the land rebounds when it finally notices that the weight of the glacier is gone, and an arctic tundra is formed. The climate warms and soils redevelop.
You can date plant and animal matter with carbon dating. Thermoluminescence can show the last time that a rock saw daylight. Zircon dating is used for older rocks.
The thrill of the day was when Jim stood on Hammond, and said – Sure looks like Chuckanut sandstone to me! – The 45 million year old soft rock, with its typical honeycombing, does not appear on the geological map but there it was, plain as day.
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March 8 2008 TNC Beach Walk with Jim Johannesen April 8, 2008 Peter Olesiuk on Pinnipeds