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Adventurer in life's bewilderness

Monday, August 22, 2011

Geology--History Emergent

I wasn’t prepared for the first words Cindy Buxton spoke as we stood on the mud flats looking up toward the homestead. “There are world class rocks on the Chilkat Peninsula that would excite every geologist in the world.”

Cindy’s authority stunned me in much the same way I find myself humbled by the raw grandeur of Alaska’s landscapes. I wanted to learn something of what formed the ground we stand on. A history mysterious to me shaped the Chilkat Peninsula, the craggy peaks on the far sides of Lynn Canal and the Chilkat River. A Haines geologist was willing to share her knowledge. In less than an hour I gained a deeper understanding of our past which I share with you in Cindy’s words:

Encompassing the intertidal flats at the head of Mud Bay, the Nelson Homestead was shaped by the rich geological and geomorphic history of the Upper Lynn Canal and the Chilkat Peninsula. All three major rock categories—igneous, metamorphic and sedimentary—are found on or near the homestead.

About 220 million years ago lava flows covered the landscape, coalescing into a thick pile of basalt. Near the end of the volcanic episode some of the lava flowed into the ocean, forming spectacular pillow basalts and other rocks similar to what one might find on the coast of Hawaii today. These lavas were then buried by about a 5-mile thick sequence of ocean sediments over tens of millions of years. The heat and pressure at such great depths caused metamorphic changes in the basalts and sediments. Over time immense forces lifted the buried conglomeration of metamorphosed materials. Huge portions were eroded away, once again exposing the lower sediments and the now metamorphosed lavas. 

Approximately 18,000 years ago at the peak of the last ice age, the entire homestead was under about 3500 feet of ice. The moving ice eroded the area into its current dramatic landforms leaving glacial striations on mountain tops and numerous large glacial erratic boulders to dot the landscape. The ice advanced again during the Little Ice Age, likely filling only the deeper valleys and then retreated 300 years ago. No longer depressed under the huge weight of the ice, the crust began to “rebound” upwards, a total of about 15 feet since 1770. Currently the rebound rate is about 0.9” per year, a rate second in the world only to the uplift in Glacier Bay which is rising at a little over 1” a year.

Two faults cut through the homestead, connecting the major parallel faults running up Lynn Canal and the Chilkat River, both of which in the past were equivalent to the San Andreas Fault. Movement on the “Mud Bay” faults is sideways rather than up and down, shifting the peninsula south of the homestead to the northwest in relation to the peninsula north of the homestead. Erosion on these faults by streams, and especially the large glaciers, formed Mud Bay and Letnikof Cove.

No earthquakes have been recorded on the small faults on the homestead, although many small to medium earthquakes are known to occur on the faults in Chilkat River and Lynn Canal. The two “Mud Bay” faults partly control the local distribution of fresh water and salt water in nearby wells. One of these faults likely funnels fresh ground water up to the artesian well on the homestead.  Many wells on nearby properties hit salty water.

Cindy’s description prompted me to renew my vows of gratitude. Whether filling water jugs at the spring, gathering wood for the wood stove or joining friends at the garden to harvest vegetables and share a potluck meal around the fire, I find myself remembering the essence of these thoughts penned by John Muir over a century ago:

“This grand show is eternal. It is always sunrise somewhere; the dew is never all dried at once; a shower is forever falling; vapor is ever rising. Eternal sunrise, eternal sunset, eternal dawn and gloaming, on sea and continents and islands, each in its turn, as the round earth rolls.”

Cindy Buxton is a geologist with 25 years of extensive experience in resource estimation and geologic mapping for a variety of mining companies. She has worked as an independent consultant since 1996. Her experience in Alaska includes the Brooks Range, Seward Peninsula and Southeast Alaska. She has a Master of Science in Geology from the University of Washington, Seattle, 1990, with a thesis, “Geology and Pre-Metamorphic Evolution of the Nome Group Blueschist Terrain, Horton Creek Area, Seward Peninsula, Alaska.” Her Bachelor of Arts in Geology, 1987, is from Rice University, Houston, Texas, with a thesis, “Field Guide to the Precambrian Rocks of Central Texas.” As a hobby she documents the local geology and leads school and community field trips under the auspices of the Takshanuk Watershed Council program, Living in the Forest Presents, to teach Haines residents and visitors of all ages about our geologic history. Her most recent geology field trip was “Volcanic Rocks of the Haines Peninsula.” She also gives occasional geology talks at the Haines Public Library.






1 comment:

Anonymous said...

Hey, Your story put the changes this place has undergone into a certain perspective - how often do I really think in those terms? Maybe the mess we're making of the world with greenhouse gases is yet another change this earth will soldier on through; life evolves. Jaybird