Waiting to Tell an Epic Tale

The Roxbury Conglomerate waited to tell its story 20 feet from where I played with my son and visited with friends at Martin Hilltop Playground in Dorchester, Massachusetts.  Rocks have a way of doing that; waiting that is. Parts of this outcrop had survived 650 million years, traveled thousands of miles and climbed probably two or three to tell an epic tale of rises and falls, separation and unification. And it was only on my way out of the park that I took the time to glimpse the gray layered rock with walnut sized chunks of white , gray and pink stone trapped within.  Each type of smaller stone told part of what it took to get this rock to a park near the south edge of Boston.

The story of these rocks begins not far from the coast of Africa.  A light colored rock with largish crystals called granodiorite indicates a fond farewell from Africa.  The dense rock that forms much of the ocean's bottom is jet black, and we call it basalt. When a new ocean tears a continent open, that basalt forming magma wends through lighter colored continent rock, forming a kind of salt and pepper blend of the two called diorite.  Chunks of the Dedham Granodiorite (an even lighter version of diorite) within the conglomerate may have formed in a prePangea split. As this island fragment of Africa set off on its own, erosion and sorting would have created new sediments. Another type of fragment, the gray Westboro Quartzite, thought to have a similar age as the Dedham rock, may have been a beach on the coast of this diverging island. The beach would have been transposed to sandstone and eventually quartzite with time and travel.  Each of these rocks, encapsulated in the conglomerate is a chapter in the conglomerate's story.

Breaking from its roots in Africa was only one small step in Avalon's eager journey to become part of North America.  After all, the ocean is not only wide; it's firm.  Unlike the water that overlies it, the bottom of the ocean is solid stone. Once Africa split, an ocean bottom's worth of material, like so much yellow shag carpet, would need to be removed to make way for Avalon's trip. But, where to put it? A convenient choice might be underneath the very crust of the Earth itself. As the continent moved west to escape Africa and join America, the ocean in its path was shoved below (in a process called subduction). This arrangement only works so well, however. Less dense ocean bottom material has trouble sinking through denser mantle (the layer of the Earth beneath its crust), and post-descent some of the slab melts and floats upward. When this melted rock makes its way to the surface, we call it a volcano. Thus was born the reddish  Mattapan Volcanics, another component of the Roxbury Conglomerate.

The large chunks of rock contained within tell their own individual parts of the story, but the conglomerate itself reveals the denouement.  Imagine standing in stagnant water, your feet squelch in the muddy bottom. In a stream, or wavy coast, mud is hustled away and what remains is sand. Speed that stream up by, say, running it through a set of miles-high  mountains, and your feet rest on well worn gravel.  All but the biggest chunks of the mountains above have been dragged well out to sea by a raging river.  How do you get mile high mountains made of an amalgam of granodiorite formed deep in a splitting continent, quartzite formed on shoreline, and volcanic rock formed in migrating islands?  You crush the whole, complicated, landscape in the binding vice of Africa and North America as they form one corner of the supercontinent Pangea.  The mountains climb, and the gravel falls.

The intervening couple of hundred million years is mostly waiting for the conglomerate and its components. Waiting to harden from riverbed into stone. Waiting for the giant mountains to be worn down to roots. Waiting while glaciers scour the rock and sediment above. Waiting for Boston and the park to be built. Biding time for the opportunity to tell an epic tale to curious visitors. Don't worry, the rocks don't mind the wait.


Hepburn, Christopher J., Javier Fernandez-Suarez, George A. Jenner, and Elena A. Belousova. "Significance of Detrital Zircon Ages from the Westboro Quartzite, Avalon Terrane, Eastern Massachusetts." Geological Society of America 40.2 (2008): n. pag. Web.

Reid, Annie. "Nature Notes 5/12/2006 - A Mighty Collision and Much Glaciation." The Westborough News. Westborough Community Land Trust, 12 May 2016. Web. 06 July 2016

Watts, Douglas. "Geology of North Easton, Massachusetts: We're Still in West Africa." Tispaquin's Revenge. N.p., 6 Feb. 2010. Web. 06 July 2016.

 Thompson, Margaret D., and O. Don Hermes. "Ash-flow Stratigraphy in the Mattapan Volcanic Complex, Greater Boston, Massachusetts." Geology of the Composite Avalon Terrane of Southern New England Geological Society of America Special Papers 245 (1990): 85-96. Web.

Everybody's Got to Eat

Life will do anything for energy. On the mid slope trail of Portland's Eastern Promenade the ruddy, oily mess that surrounded me made that abundantly clear. This particular portion looks like the effluent pond of a pre-Clean Water Act chemical plant. In one direction water seeps out of the ground, somewhere between the color of blood and orange finger paint. In another, a rich, rainbow sheen texturizes the look of a shallow pool, bringing to mind the "Dump No Waste, Drains to Lake" stencils that pop up around water bodies. Despite the look, I know better. I've seen similar scenes in Acadia National Park, an area protected and remote enough that dumping just doesn't seem worth it, even for the most villainous human polluters.  Bacteria, however, are another story.

When you get hungry, you might grab an apple and chow down. Your body tears apart the weak bonds of the apple's sugars, leaving a soup of carbon, hydrogen and oxygen atoms. These atoms don't like the single life and when your lungs immerse them in a bath of even more oxygen, the hungry oxygen atoms snap up carbon and hydrogen to create your two favorite molecules: Carbon dioxide and water. You might think it's the sugar that gives you the energy, but, really, your energy comes from building the strong bonds in the compounds you exhale.

The polluters on the East End work the same way: they take up weakly bonded or unbonded atoms and snap them into strong bonds to create energy. Of course their chemical soup is completely different than yours and mine. Instead of carbons and hydrogens, they take up the fourth most common element in the Earth's crust: iron.  Like carbon and hydrogen, iron isn't a big fan of going it alone, but deep underground there isn't much of a selection of partners. But, as ground water flows to the surface, it drags with it lonely iron ions, inviting them to a chemical party that they wouldn't have had access to in the subterranean world.  Up here, oxygen is a near perfect mate. The bacteria are the E-Harmony  of the chemical world, speeding up the matchmaking process and reaping the energy benefits when sparks fly. When oxygen joins with iron, the strong bonds snapping shut powers the life of the bacteria.  Like you and I exhale carbon dioxide and water, bacteria pumps out these red-orange iron oxygen compounds, called iron oxides.

That explains the red seeps, but what about the oil on the water's surface?  It's worth thinking about what oil is.  Generally, oil is the leftover parts of simple organisms that lived a long time ago. In a way, the oily sheen is donated by a material not too different than the fossil fuel. Bacteria, including those that metabolize iron, have a short lifespan. The creation of iron oxides provides the energy needed to power more and more of this simple life form. When members of the community die, their parts float on the surface. The surface film of broken up bacteria creates the same rainbow effect as their prehistoric counterparts that form petroleum.

These iron bacteria may have an impact on our visual environment.  But it's not fair to call them polluters.  They're just trying to live the same way they have for almost a billion years.  To do that means eating, breathing and dying, just like you and me.

Clark, M. S. (2015, October 16). What is Oil? Retrieved April 16, 2016, from http://www.sjvgeology.org/oil/oil.html

Ilbert, M., & Bonnefoy, V. (2013). Insight into the evolution of the iron oxidation pathways. Biochimica Et Biophysica Acta (BBA) - Bioenergetics, 1827(2), 161-175. doi:10.1016/j.bbabio.2012.10.001

Wartinbee, D. (2010, March 24). Science of the Seasons: Yellow boy bacteria has people seeing red. Retrieved April 16, 2016, from https://redoubtreporter.wordpress.com/2010/03/24/science-of-the-seasons-yellow-boy-bacteria-has-people-seeing-red/