Wednesday, August 14, 2013

A Piece of Ira: Unwrapping Turbidites

The first mistake many potential geology nerds make is to assume that any given rock doesn't have an interesting story.  If they didn't make that mistake I would be sitting here reading their blogs rather than writing my own.  What makes this error so common is that many of these stories are hidden, or written in languages that we can't read.  My friend Jenn brought me a wonderful story from Ira Mountain in a flat sparkly package.

Aww...Schist.  The sparkly covering on this rock is
ancient ocean bottom heated in a continent colliding oven.
Nothing wraps a rock in sparkles cheaper than mica.  Gold and Silver requires hot magma to transport heavy metals from deep within the Earth, where denser metals tend to reside.  Mica is a working man's sparkle.  It's the mud under your fingernails, or more likely the mud at the bottom of the ocean.  Fine-grained particles, like crushed up mica, drift through water under the power of the slightest current. They settle only when wave power is nearly nil.  In other words the deep dark ocean.  This mud is composed of a few things, but mica contributes heavily to the mix.  Of course, you'd never know it because the particles are so small.  Finishing the shiny package means burying the rock and crushing it between a continent, say North America, and a microcontinent, perhaps a prehistoric piece of land that now makes up shoreline on Maine, Greenland, the United Kingdom and Scandinavia called Avalonia. The heat and pressure caused small particles to migrate toward one another creating the large sparkles we now see.

Quartzite Revealed.  Notice the blockier texture on the
bottom half of the rock.  There is a thin layer of schist
below the quartzite,
Of course, you can't just wrap wrapping paper.  A glance between the layers of gray-green mica reveals a reddish blocky layer with a sugary appearance that has been compared to the texture of a gumdrop. This is where the surprise and the story lie.  Quartzite, unironically, is made out of quartz. Because quartz won't break down into to tiny pieces the way mica does, its grain size tends to bottom out at a 0.0025 inch.  The mass provided by this bulk causes even fast moving water like that on a wavy coastline to drop sand grains like they were hot, much to the enjoyment of southern Maine's beach-goers.  Crush that sand in the vise of continents and you get quartzite.

So what brings the unmoving abyss of the ocean bottom next to the active coast.  One possibility is time.  Sea level has fluctuated throughout geologic time.  To whit, the Hannaford in West Falmouth was built on ocean bottom from the ice age, and taking I-95 north of Gray reveals sandy roadsides that may be beaches of the same time period.  This sort of shift in water level is certainly capable of creating a rock that transitions from mudstone to sandstone and back again, or post crushing, a quartzite sandwich, with schist bread.

But there is another possibility.  The Carrabassett Formation, out of which the north half of Ira Mountain has been cut, is known for its turbidites.  Imagine an underwater ridge.  The top of the ridge, exposed to the movement of waves and the final surges of upland streams, plays host to hefty sediments like the aforementioned sand.  The bottom of the ridge, protected from the rigamarole, cradles the tiny clays. Then something happens.  A stream changes course.  A rogue wave mixes things up.  The ridgetop sediments, perhaps already precariously perched, fall down.  The movement stirs up everything, creating a stew of water, sand and clay.  As time passes the turbid water clears again, first dropping the heavy sand, then setting down the lighter clay.  This process would repeat again and again over the life of the ocean and the ridge until these layered sediments became stone.

Which of these possible histories is true is unclear.  Maybe this detail of the story is unwritten, or maybe I just haven't learned how to read this piece of the language.  It is the hope that we might know more, that there is some unwrapped gift, that keeps me looking and keeps me learning.

Adams, Dennis. "About Beach Sand."Beaufort County Library. Beaufort County Library, Web. 14 Aug. 2013. <>.

Dorais, Michael J., Robert P. Wintsch, Wendy R. Nelson, and Michael Tubrett. "Insights Into The Acadian Orogeny, New England Appalachians: A Provenance Study Of The Carrabassett And Kittery Formations, Maine." Atlantic Geology 45 (2009): 50-71.

Hanson, Lindley . "The many expressions of a New England formation." Vignettes: Key Concepts in Geomorphology. SERC, Web. 15 Aug. 2013. <>.

Schieber, Jürgen . "Sedimentary Structures." Indiana University. Indiana University, n.d. Web. 15 Aug. 2013. <>.

Weller, Roger . "Schist Photos." Virtual Geology Museum. Cochise College , 24 May 2013. Web. 15 Aug. 2013. <

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