Saturday, October 10, 2020

West of North Beach, Port Townsend

With cool early October nights, a flow of marine air from the ocean and perhaps some enhancement from California wildfire smoke, I had a very foggy venture along the shore of the Strait of Juan de Fuca west of North Beach Park in Port Townsend.  

Moisturizing air

The eroding shoreline bluffs allow for great cross section views of glacial related sediments.

Three units exposed on bluff slope from bottom to top: Glacial till, glacial marine drift and emergence deposits

The till was deposited directly by glacial ice when glacial ice from the high coast mountains of Brish Columbia flowed across the area between 18,000 and 14,000 years ago. 

The glacial marine drift was deposited when the ice thinned towards the end of tha last glacial period. The thick ice mass had depressed the local land surface hundreds of feet and when the ice thinned the area was inundated by sea water with glacial ice floating on the sea water. The melting floating ice rained sediment onto the sea floor forming a deposit called glacial marine drift. 

The glacial marine drift is compositionally the same as till, but the till has been compacted by the overriding glacial ice. The glacial marine drift is hard as well as it self compacted by wetting and drying. But that self compaction forms fine fractures within the hard drift.

Fractured and cracked glacial marine drift

The land rebounded after the ice left, and the glacial marine drift emerged from the sea. The upper layer of the drift was reworked by waves and tides for a brief period and the finer silts and clays were washed away leaving the gravel and cobbles behind at the top of the marine drift as emergent lag deposits. 

A bit further to the west another unit presents itself.

Chaotic mix of silt blocks and occasional blocks of glacial till embedded in gravel  

These deposits are great example of and a reminder of the force of under ice water flow. The Juan de Fuca ice lobe and the Puget ice lobe were melting as they advanced and retreated from the area. Much of that melt water would have been flowing under the ice as a confined and powerful hydraulic force that would tear up the underlying sediments. The shoreline reach between North Beach in Port Townsend and just past Middle Point to the west is one of the best exposures I have seen. Indeed it is hard to imagine a better one given the long line of well exposed geology on these eroding bluffs. 

This shore reach is fairly well visited. Even on a foggy chilly morning I encountered others venturing along the shore. Many of the beach walkers are pretty locked into the beach looking for various treasures. As one heads west the beach sands and gravels present polished glass and is locally referred to as Glass Beach. The glass is derived from an old dump site as well as some less concentrated dumping including vehicles.


Unknown said...

Interesting! So much to see if we have the knowledge.

Hollis said...

oops, that was from Hollis, there was a login error. Best wishes

Anonymous said...

Have looked at the bluff many times. Thank you for the little Geology lesson about it.

david said...

Love to hear about local Ice Age geology.