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.

A Tale of Two Conglomerates: Chapter 2

This is Chapter 2 of a 3 part blog post.  Click here to read part 1.

Chapter 2: The Matrix
Conglomerates share there stories in several ways.  The pebbles that fall together to make the rock tell a story of what came before.  The stuff that holds the rock together - the matrix - speaks about what was happening when the rock came together.

Four hundred and fifty million years ago, Mars Hill would have been pretty close to the Equator.  This time period also happened to be when coral were distributed widely around the world.  These facts were unknown to me at the time I first visited Mars Hill.  What I did know, however, was that Mars Hill was not far from a town called Limestone.  I looked at those clasts, and I looked at the stuff that held them together (called the matrix in the geology world), and I wondered.  I have since broken my piece of conglomerate and, logically, dropped it in vinegar.  The neat thing about limestone is that you don't have to wonder for long.  Dropping limestone in vinegar causes a reaction between the acetic acid of vinegar and the limestone base, causing carbon dioxide to fizz off.  Soon after the rock hit the bottom of the mug, bubbles started rising to the surface of the vinegar.  Four hundred and fifty million years ago, about the same time that ocean bottom rock was being torn apart, a coral reef, not far from Mars Hill was breaking down as well.  While the majority of this calcium carbonate piled up on flat ocean bottom, creating the substrate for Aroostook County's potatoes, some followed the flow of water over some sort of cliff into some kind of deep water canyon allowing shale pebbles and limestone mud to mix together.

The pebbles that make up the Mount Battie conglomerate are held together by something else.  500 million years ago, Mount Battie was in a part of the world not very likely to host coral.  The Avalonian microcontinent, perhaps similar in form to today's Japan, was on the bottom part of the world, below the 60th parallel.  The matrix here, insoluble in acid, is quartzite, indicating a sandy environment.  Layering at the site indicates the sand gathered in an ocean basin, where wave action swept away most of the smaller sediments, leaving behind sandstone pebbles in a matrix of sand.  This shore line would have been a bit cold for developing coral reefs, excluding the development of limestone in the area.  This gravelly beach the stage for our future coastal mountain.      

Modern World Coral Reef Locations - Credit: NASA

Berry, Henry N., IV. "The Bedrock Geology of Mount Battie, Camden." Maine Geological Survey: , Maine. Maine Geological Survey, 19 Apr. 2012. Web. 01 July 2013. <http://www.maine.gov/doc/nrimc/mgs/explore/bedrock/sites/jul01.htm>.

Scotese, C. R. "Earth History." Plate Tectonic Maps and Continental Drift Animations. PALEOMAP Project. Web. 01 July 2013. <http://www.scotese.com/earth.htm>.

Wang, Chunzeng, Gary Boone, and Bill Forbes. "Geology of Mars Hill Mountain and Vicinity." Http://goaroostookoutdoors.com/. Web. 1 July 2013. <http://goaroostookoutdoors.com/sites/default/files/trails/maps/mars_hill_geology2.pdf>.