Through the Past and into the Future: A View from the Harpswell Cliffs

The cliff walk in Harpswell lives up to its name. The trail makes a turn and dumps you out on a view worth far more than the effort required to hike to it. While it didn't take me long to reach the top of the 150 foot cliff, the rock itself would have taken far longer to make the climb. The rock is schist - the glittery remains of mudstone that has been crushed and baked in Earth's oven. The mud itself piled up in some prehistoric ocean bottom, perhaps at a rate of a foot every thousand years. Peering down the 150 foot cliff to the estuary below, I imagined a stream tediously cutting through millennia worth of deposition, time traveling further and further into history. This erosion might open a window into a time period 150,000 years before the rocks on which I stood formed. As it turns out the chasm was a portal through time, but the time machine traveled into the future.

Around 490 million years ago, a slab of continent we call Avalon near the South Pole cleaved itself from its parent, a supercontinent called Gondwana.  As it journeyed northward it developed a predictable series of layers.  An early layer became our cliff. Not long after it left the Antarctic Circle mud rained down on the ocean floors of the microcontinent's coast. It accumulated for that 150 thousand years and longer.  As the microcontinent scrolled past 40 South, 445 million years ago, volcanoes laid new rock on top of old. This was, in turn, buried by ocean bottom. The new ocean floor was topped by one final, explosive, volcanic eruption, the icing on a layer cake of rock that was a testament to the long journey. 

A lot went on to make the Harpswell Cliffs time portal.
First the layers of rock were laid down flat (with the
schist of the cliffs in blue and the volcanic rock across
the estuary in red).  Then the layers cracked and stacked.
Next they bent into wavy layers.  Finally the tops were shaved
off by erosion, exposing our cliffs and the peninsula below.

In an orderly world, where progress is constant, it is obvious that the rocks of the Antarctic circle would never be seen again. Instead they would be buried deeper and deeper. But these rocks lived in a world where journeys end. The unstoppable northward force would eventually meet its harbor, the immovable North American continent. This collision was massive. Pieces of continent were flung and tumbled, like ice onto a spring shore.  Avalon cracked and its parts were thrust on top of one another.  Now it was that final explosive eruption, the icing on the cake, that had been interred by its own past.

The compression of this part of the Earth continued. The heat and pressure of the collision would transmogrify the fine grained mudstones into glittery schists. The layers would flex into a washboard of ridges that make southern Maine's coastal islands and peninsulas so multitudinous, lifting the schist of those cliffs to their present height and higher.  The problem was that gravity abhors a ridge. Ice, water and good old falling did their best to even off the tops of the rolling ridges. Sanding of those tops meant revealing the bowed edges of the cake layers.  

It may have been a glacial flood, a stream funneled by a weak layer of rock, or just plain luck that cut through those last 150 feet. Whatever it was gave me a window through one hundred and fifty thousand years of mud to a blue estuary below. Looking across the water revealed not a predictable past, where time moves ever forward. Instead, I looked into an explosive future where, given enough time, the world could be turned upside down.

Liu, Dennis. Earthviewer. Computer software. Vers. 1.1. Howard Hughes Medical Institute, Jan. 2013. Web. 11 Apr. 2015

Hussey, Arthur M., and Henry N. Berry. Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine. Augusta, Me.: Maine Geological Survey, Dept. of Conservation, 2002. Print.

Druyan, Ann, and Steven Soter. "The Clean Room." Cosmos: A Spacetime Odyssey. Fox. 20 Apr. 2014. Television.

Say Goodbye to Graptolites...

Graptolite fossils from Presque Isle

It's hard to get sentimental about graptolites. The fossil of what's actually a colony of tiny animals looks more like a small saw blade than anything you might start a preservation campaign for.  It's probably for the better, too. A successful "Save the Graptolites" crusade would have preserved a world where reptiles and pine trees would never have thrived. In Silurian era Presque Isle, however there would have been no need to improve conditions for graptolites. They were already perfect.  

In a low outcrop not far from downtown Presque Isle, the pale rock feels gritty, but not coarse. This type of sediment is not the product of a deep ocean or a beach, but the sweet spot in between.  It's born in an area where prevailing winds could blow off the warm top layers of an ocean, and reveal the cold nutrient rich waters below.  Perhaps inconsequential to you or me, but life changing to a graptolite.

Certain shards of the rock slab host the shallow impressions of graptolite rhabdosomes, basically an apartment building for tiny animals called zooids.  In Silurian time the complexes either rooted themselves in coastal sediment or floated in masses at the surface.  The zooids may have reached out of the many holes in these colonial homes to grasp at and eat phytoplankton from the coastal waters, enriched by the upwelling of vital nutrients.  

In a time when Caribbean-like islands stood sentry on the coast of North America habitats like this would have been commonplace in Maine. But, Maine was changing. Three hundred eighty million years ago the tectonic block that serves as our current coast, Avalon, docked with Maine. The conjunction crumpled the graptolites' ocean habitat.  Driving fossil types of graptolites from fourteen  in the Silurian to one after the collision. Eighty million years later the jaws of Pangea snapped shut.  The huge landmass was inhospitable to the aquatic creature and by the time it had formed the graptolites were extinct.

The watery Silurian period was ideal for graptolites. The changing environment drove them out of Maine, and then to extinction.  A sad moment, perhaps. But, the transition to dry land had some advantages, too.  Plants left the coasts, invading Pangea, and in the process became trees. Amphibians bravely abandoned their shrinking watery habitats and evolved into reptiles. Graptolites didn't make it, but maybe that's okay. Perhaps their loss is our gain.

Dickson, Lisa, and Robert D. Tucker. Maine's Fossil Record: The Paleozoic. Augusta, ME: Maine Geological Survey, Dept. of Conservation, 2007. Print. 

Koren', T. N., and R. B. Rickards. "Extinction of the Graptolites." Geological Society, London, Special Publications (1979): 457-66.

"Graptolites." Common Fossils of Oklahoma. Sam Noble Museum. Web. 20 Dec. 2014.

The Outcrop in the Crystal

It's my 20th post!  This blog post is the first of several that will highlight an outcrop that drove my interest in Maine geology.  If you're looking for the outcrop it's on Rte. 115 on the Gray/ Windham border.  Or check it out on the Sphere app.  

The outcrop told the story of Maine.  The white rock, a granite, bubbled up when Africa's coast collided with our own 450 million years ago, sealing Maine into the Pangeaic interior.  The black basalt resulted from one of the volcanoes that once again cleaved the supercontinent in two when Pangea split.  The glittery schist told the tale of a muddy ocean bottom that predated both two igneous rocks.  But how did the green rock get there?

There were obvious clues.  The parallel stripes of white, green, and sometimes even gray told tales of piling layers.  The fact that these layers took turns with schist, a mudstone altered by heat and pressure, as one scanned the outcrop from left to right suggested that it, too, formed in that muddy sea.  Other facts didn't add up.  The tough rock scored glass.  Most of the usual sedimentary suspects wouldn't do that; unmetamorphosed mudstone and limestone were just too soft.  Their metamorphic progeny, schist and marble would have crumbled as well.  Sandstone, and its postbear quartzite, would do the job, but the look wasn't right.  The rock lacked the granularity of sandstone, and the sugariness of quartzite.

The truth became clearer as I began to research Silurian time, a period between 416 and 444 million years ago, in which the green rock formed.  The ocean bottom that it gathered in was flanked on three sides.  To the north lie North America, to the south, a hunk of land that would become Maine's coast. To the east, Western Europe plugged another opening preventing the flow of currents.  The phlegmatic basin, just south of the equator, became poor in oxygen, but rich in life.  Lacking our usual breath of life, the creatures of the sea resorted to extreme measures - consuming sulfur.  

The chemistry of this bounded sea played an important role in the green rock's formation.  As a coral reef, in an aerated ocean, degrades, it forms limestone, which is composed of the elements calcium, carbon and oxygen.  Our sluggish sea would have produced a similar product, with one small difference: the sulfur consumption exchanges some of the sediment's calcium for a new element - magnesium.  The rock it forms goes by a different name: dolomite.

Even dolomite doesn't have the strength to scratch glass.  The rock would require one final transformation.  As plates moved, the basin grew smaller, and then disappeared.  The formation of Pangea spurred an influx of fluids: some large, like the bulb of magma that formed the white granite, some less immense, like the infusion of quartz and water that flowed through the ancient ocean bottom.  This liquid sought out any channel it could access, including spaces in the schist, and the dolomite.  The schist proved passable, but inert.  The dolomite was reactive.  The heat and chemicals in the flowing fluid released carbon and oxygen from the dolomite (as carbon dioxide).  Some of the quartz stuck around, providing the white layers of the rock.  The green crystals, a mineral called diopside, kept the calcium and magnesium of the dolomite and replaced the CO2 with silicon and oxygen from the quartz.  

The outcrop is a testament of Maine's geological history.  It documented the marine roots and the accordion push-pull of continents.  The green diopside crystal, a fractal of that outcrop, records the deoxygenated ocean bottom in its calcium and magnesium.  It further tells the tale of the pushing and pulling continents in its silicon and oxygen.  The green crystal, a mere fragment of the outcrop, tells the outcrop's entire story.

Bickle, M. J. , H. J. Chapman, J. M. Ferry, D. Rumble, and A. E. Fallick. "Fluid Flow and Diffusion in the Waterville Limestone, South—Central Maine: Constraints from Strontium, Oxygen and Carbon Isotope Profiles." Journal of Petrology 38.11 (1997): 1489-1512. Print.

Ferry, J. M.. "A Comparative Geochemical Study Of Pelitic Schists And Metamorphosed Carbonate Rocks From South-central Maine, USA." Contributions to Mineralogy and Petrology 80.1 (1982): 59-72. Print.

Fischer, Dan , Tammy (Yue) Liu, Emily Yip, and Korsen Yu. "The Silurian Period."The Silurian Period. University of California Museum of Paleontology, 5 July 2011. Web. 6 Dec. 2013. <http://www.ucmp.berkeley.edu/silurian/silurian.php>.

Hussey, Arthur M., II, 1996, Bedrock geology of the North Windham 7.5' quadrangle, Maine; Maine Geological Survey (Department of Conservation), Open-File Report 96-16, 6 p.

Helmholtz Centre for Ocean Research Kiel (GEOMAR) (2012, June 7). How does dolomite form?. ScienceDaily. Retrieved December 8, 2013, from http://www.sciencedaily.com/releases/2012/06/120607105815.htm

Wilde, Pat , William Berry, and Mary Quinby-Hunt. "Silurian Oceanography."Marine Sciences Group. University of California, Berkeley, n.d. Web. 8 Dec. 2013. <http://www.marscigrp.org/sil91.html>.