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.