Geoducks

I have spent a fair bit of time on the high bluffs along the east side of Port Discovery. During some of those ventures a small boat has been off shore in a stationary position with the sound of engines running. At other times the boat is simply anchored near the end of an unpaved road adjacent to a cuspate shore form.  

Landslide complex with cuspate shore form in distance and anchored boat

Port Discovery with Olympic Range in the distance

The boat is used by divers for harvesting a lease tract subtidal geoducks. Dave Williams has an excellent write up of the history and management of the resource (pugetsound.org/geoduck-clams). The shellfish can be harvested during very low tides without diving.  (b-roll:-geoduck-beach-harvesting-aquaculture). However, a fair bit of the harvest is via dive boats at lease sites (youtube.com Geoduck Diving Harvest). The geoduck fishery is managed by Washington State Fish and Wildlife as well as the owner of the subtidal lands, Washington State (dnr.wa.gov/wild-geoduck-fishery)

The first time I tried geoduck was via a gift from shellfish business I did some geology work for. I have since dug for them a few times during very low tides. They are a fun prize to pull up out of the sand. One big goeduck is more than enough for a meal. But those big geoducks take many years to grow. As their value has increased management of the resource has become more important.    

From the Columbia River to The Fraser River in 1824 - John Work

I roughed in the route of a 1824 Hudson Bay Company expedition from Fort George, the former Fort Astoria trading post, to what was later to become Fort Langley via Journal of John Work, November and December, 1824 via T.C. Elliot, 1912. 

Part of the scheme was to find the location of a trading center fort on the Fraser River that could receive export goods (furs) from the interior of western North American. Hudson Bay Company (HBC) was already concerned that the lower Columbia would end up in American possession. In part because Fort George was originally an American establishment. In addition, at that time navigation from the sea to the lower Fraser was far superior to the harrowing crossing of the entrance to the Columbia. However, while the Fraser River was a better sea access point, the Fraser River was not a good river transit route.  While a fort ultimately was built, Fort Langley, the Fraser proved to be an impossible navigation route from the interior.

The journal is a window to a different time, and for that matter a different place. I have been fascinated by the diversity of the people and interactions that took place in the brief period from 1790 to 1825. The assembled team for the venture clearly had a pretty good idea of the route they should take. The team included several Hawaiians (Islanders) as well as an Iroquois hunter and his slave.   

Note the time of year they traveled, November-December. While it does rain in November and December in western Washington, the outer coast and the southwest part of the state gets 'heavy rain', a term that Mr. Work used in his journal of his venture (correct spelling of his name was Wark). This was a hardy group. I contemplate how I would have performed portaging across wet ground and along shorelines, rowing for many miles, and camping in the rain, the heavy rain.   

Rosario Fault on Crane Island north shore

During a work venture in the San Juan Islands I got a nice look at the bedrock along the north shore of Crane Island. Crane is a rocky island between the southwest end of Orcas Island and the northwest shore of Shaw Island. The ferry between Orcas and Friday Harbor passes along the south shore of Crane, but on this venture I was in a small boat and we ventured along the north shore. 

Crane Island is mapped as Constitution Formation on both the Bellingham 1:100,000 map (Lapen (2000) and the Roche Harbor 1:100,000 map (Logan, 2003). And both maps denote the Rosario Thrust Fault immediately to the north of Crane Island 

Portion of Bellingham Quad geology map (Lapen, 2000)

KJmc = Constitution Formation, JTRmcto = Orcas Chert, pPscg = Garrison Schist, pDit = Turtleback Complex, Qgd = glacial drift. Heavy black lines with saw teeth are thrust faults with teeth on upper plate.

Portion of Roche Harbor Quad geology map (Logan, 2003)

The Rosario Thrust is where the Constitution Formation has been faulted up over the Orcas Chert. The fault zone is a zone of highly sheared rock that in places include slivers and slices of other formations caught up in the fault zone such as the Garrison Formation denoted by Lapen (2000) on Shaw Island south of Crane Island. 

My observations of the fault zone exposed along the base of the steep north slope shoreline on the north side of Crane were consistent with other areas where the Rosario Trust Fault is well exposed along the shore near Rosario on Orcas Island (hence the faults name) and along the southwest shore of San Juan Island. The shoreline areas typically offer the best exposures of the fault zone.     

Highly sheared rock within the Rosario Thrust along the north shore of Crane Island

Brecciated and sheared bedrock at water line

Massive block of Constitution Formation above shear zone

On the uplands of Crane Island, the bedrock is generally massive metamorphosed sandstone of the Constitution Formation.  

Typical massive silica-rich metamorphosed sandstone

Orcas Chert is primarily composed of ribbon chert, alternating layers of silica rich 1 to 2 inches thick with thin layers of shale. The chert is derived from the accumulation of silica diatoms that accumulated on the ocean floor. The shale was derived from fine dust and silt that also reached the ocean floor. The layering developed after deposition when the ooze of silica rich and silty sediment was compacted. The age of the Orcas Chert is between 180 and 280 million years old. The Orcas Chert is part of terrain consisting of the related Deadman Bay Volcanics, which are ocean floor pillow basalts that are somewhat older than the Orcas Chert and likely formed the basement that the silica rich sediment that formed the chert unit was deposited on. The original depositional setting was an ocean floor area far from North America. The formation was moved to and accreted to the edge of North America via plate tectonics. During accretion the Orcas Chert was metamorphosed at high pressures.

The Constitution Formation was also originally an ocean floor assemblage; however, this unit is on the order of 130 million years old. The Constitution Formation consists predominantly of metamorphosed fine sandstone derived from a volcanic arc. There is some chert and basalt in the unit as well. This formation was also accreted to the edge of North America. A lack of older aged sediment, North America sediment, suggests the sediment may have been derived from an ocean island arc. 

The Rosario Thrust Fault and the juxtaposition of the Orcas Chert and Constitution Formation took place after accretion -- note that the younger Constitution is thrust over the older Orcas Chert.    

The map pattern shows that Crane Island is klippen of Constitution Formation on the underlying Orcas Chert. Parts of the Constitution Formation have been stripped away leaving the Constitution Formation as an 'island' on the Orcas Chert and as well as the rock formation that makes up most of Crane Island.