Saturday, August 31, 2013

The Ocean Bottom on the Mountain

Ernest Hemingway is rumored to have won a bet by telling the world's shortest story: "For Sale: Baby shoes, never worn."  As I sat on the side of Dorr Mountain, waxing geologically to my brother-in-law, he pointed out to me that the story I was telling was really much shorter than the words I was using.  Perhaps a shorter telling would have gone something like this: "Ocean bottom found on a mountaintop."

Hemingway's story was all about what it implied.  So what is implied by a four-foot boulder of ocean bottom rock?  Time.  Ignoring that this rock was part of a much larger formation, we can conclude that these four feet must have piled up over time.  One resource suggests that clay, likely once a major component of this chunk of sea floor, gathers at 10mm every thousand years.  This isn't a rock; this is a time lapse of 1.2 million years of arduously slow settling of clay.

So what about the mountain itself? The depths at which granite is formed are measured in miles (kilometers really, but this is America, right?).  That means that the bulb of granite that makes up most of Mount Desert Island once rested deep under the Earth's surface; now Dorr Mountain stands nearly a quarter mile above sea level.  Over the hundreds of millions of years since that deep magma cooled, wind, water and ice have excavated the remains of an ancient collision between North America and the ancient land mass on which MDI rests.

A look at the sea bottom rock implies something more. Several flat faces, on many planes, give evidence of glaciers. The glacier that plucked that rock from its original bed dragged the behemoth across the Earth's surface like cheese across a grater. The chaotic innards of the glacier occasionally flipped the mighty stone, smoothing yet another face.  The glacier, perhaps a mile tall, would have both ripped at Dorr, and been slowed by it. As the ice layed the mountain low, it may have slowed sufficiently to drop some of its load - seafloor included.

Hundreds of thousands of years of piling clay, miles of erosion, and an ice sheet a mile thick, all whispered in the language of rocks.  Scenes like this, like the glacier that came before them, cover Maine. Millions of shorts stories, implying so much more.


Ansley, J. E. 2000. The Teacher-Friendly Guide to the Geology of the Northeastern U.S. Paleontological Research Institution, Ithaca, NY.  

Hardcastle, Tim . "Pelagic clay ." Historical Geology- Wikibooks. Wikibooks, n.d. Web. 1 Sept. 2013. <http://en.wikibooks.org/wiki/Historical_Geology/

Miller, Robert B. , Scott R. Paterson, and Jennifer P. Matzel. "Plutonism at different crustal levels: Insights from the ~5–40 km (paleodepth) North Cascades crustal section, Washington." The Geological Society of America Special Paper 456 (2009)


Petford, N. , A. R. Cruden, K. J. W. McCaffrey, and J.-L. Vigneresse. "Granite magma formation, transport and emplacement in the Earth's crust." Nature408 (2000): 669-673. Print.

"Dorr Mountain : Climbing, Hiking & Mountaineering : SummitPost." Climbing, Hiking, Mountaineering : SummitPost. N.p., n.d. Web. 1 Sept. 2013. <http://www.summitpost.org/dorr-mountain/416282>.



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. <http://www.beaufortcountylibrary.org/htdocs-sirsi/beachsan.htm>.

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. <http://serc.carleton.edu/vignettes/collection/25133.html>.

Schieber, J├╝rgen . "Sedimentary Structures." Indiana University. Indiana University, n.d. Web. 15 Aug. 2013. <http://www.indiana.edu/~geol105/images/gaia_chapter_5/sedimentary_structures.htm>.

Weller, Roger . "Schist Photos." Virtual Geology Museum. Cochise College , 24 May 2013. Web. 15 Aug. 2013. <http://skywalker.cochise.edu/wellerr/roc




Saturday, August 3, 2013

The Long View: The Bones of Casco Bay

In his book, Roadside Geology of Maine, D.W Caldwell compares the islands of Casco Bay to the bones of a hand.  The view from the dock on Mackworth Island provides a view of these bones.  Like paleontologists we can use these bones to tell the stories of the past.

The view from Mackworth Island pier.  The rock that comprises farther islands
formed further in the past, because the land bows upwards.  The space between
islands tells us there was weaker rock there that has since eroded.
The bones were not always the long linear ridges you see before you.  In fact 470 million years ago you would have been looking at a flat landscape lain down by volcanos and the deposition of sediment.  A collision 400 million years ago changed all of that.  If you stood on the coast in Falmouth Foreside you'd be standing on the crest of a wave created by the collision.  The rocks between this apex and the outward islands were folded into a sine curve (a syncline to a geologist) as the microcontinent Avalonia tried to cuddle up closer to prehistoric North America.  The view from the pier provides a look over the trough of this wave and up to the next crest.

A representation of a syncline.  As you look across the bay imagine
standing on the flat edge of the gray near the anticline.  You
look forward past younger rock (brown) toward older (light tan)
While the wave still persists, it is not all it once was.  400 million years of water, ice and wind have shaved off a few pounds.  As you might expect the crests have lost the most weight.  This means the younger rocks on the peaks have been removed, allowing us a look at the older rocks below.  For this reason the farthest islands are the oldest rocks.  Effectively, as you look across Casco Bay, you look back in time.  So, what do you see?

It might be helpful to think of there being two rows of islands - The Great Diamond row and the Peaks Island row.  The back end of each of these islands is made of volcanic rock.  Volcanos form from melted rock and nature makes that happen by plunging older rock beneath Earth's surface.  This tends to happen when a continent (Avalonia) pushes its way across the Earth, submerging ocean bottom as it goes.  The two Atlanticward sides of these islands represent periods of time when Avalonia was encroaching on North America, but one didn't follow the other immediately.

These time periods when Avalonia was hauling across the world were punctuated by peaceful times of rest and erosion.  The front half of the Peaks Island Row, which includes Long Island is made up of the sediments that formed as the first film of volcanic rock broke down.  These sediments would have continued to pile up until volcanos started pumping out lava again.  When Avalonia resumed its movement, the volcanoes recommenced spewing, and the back half of Great Diamond and all of Little Diamond came into being.  When the conveyor belt stopped again, erosion and deposition started anew.  Some layers were tougher, some were weaker.  The strongest became Cow and Chebeauge
Islands and the front half of Great Diamond.  The weakest were torn apart by glaciers or washed away by streams.  Portland Harbor, which divides Portland and South Portland formed as the Fore River provided just this sort of differential erosion clearing schist and limestone from between banks of harder volcanic rock.

The bones of Casco Bay are what's left after 400 million years of erosion have cleaned off the flesh of weaker rock.  These remains tell the story of Avalonia's delivery to the shore of North America, and it can all be seen from Mackworth Island.

Caldwell, Dabney W. Roadside Geology of Maine. Missoula, MT: Mountain Pub., 1998. Print.

Hussey, Arthur M., and Henry N. Berry, IV. "Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine." Maine Geological Survey: Bedrock Geology Bath 100K Report. Maine Geological Survey, 1 Feb. 2008. Web. 03 Aug. 2013. <http://www.maine.gov/doc/nrimc/mgs/explore/bedrock/b42/casco.htm>.

Marvinney, Robert G. "Simplified Bedrock Geologic Map of Maine." Map. Augusta, ME: Maine Geological Survey, 2002.

Osberg, Philip H., Hussey, Arthur M., II, and Boone, Gary M. (editors), 1985, Bedrock geologic map of Maine; Maine Geological Survey (Department of Conservation), scale 1:500,000

"Maine Geologic Map Data." Maine Geologic Map Data. 05 Apr. 2013. Web. 03 August 2013.