Friday, June 28, 2013

A Tale of Two Conglomerates: Chapter 1

A name should tell you something.  When it comes to rocks this is generally the case.  In sedimentary rocks, the name tends to give away the energy of the sedimentary environment.  Sandstone forms wherever sand might gather - like a beach with strong waves or a fast moving stream.  Shale evolves where clay might gather, say the bottom of the ocean.  Conglomerate, along similar lines, is birthed where gravel collects - perhaps a raging river or a bar in a particularly active part of the ocean.  The name conglomerate tells you that there was enough energy to carry rocks there, but the rock tells you a lot more.

Last summer I visited two conglomerate outcrops; one was Mount Battie in Camden, the other Mars Hill Mountain in Mars Hill.  These two rocks couldn't be any different while sharing the same name.  Each rock tells this history of its own place in three distinct chapters.

Chapter 1: Clasts
Mount Battie Conglomerate - Round Quartzite Clasts Circled
One neat thing about conglomerates is there are two sets of rocks with stories to tell: the conglomerate itself, and the rocks parts that are cemented together within it.  The rock parts (or in geology speak, clasts) at the two sites are surely at odds.  The Mount Battie conglomerate is made up of rounded, light colored clasts about the size and appearance of a pale gray gumdrop.  The gumdrops are quartzite.  If we could turn back the clock we would see that these chunks (like all quartzite) were once sandstone.  Imagine a wave rocked ocean off the coast of some forgotten continent.  Sand piles up (pure sand; we know this because the quartzite is so light in color) and eventually, chemical intrusion turns it from packed sand to sandstone.  Once loose sand, now solid rock, as they say, even this shall pass.  Perhaps the land gets raised and rivers cut through.  The slab of rock, once a beach, is broken by time and water into smaller pieces.  As the water courses the rocks get rounder and rounder.

Mars Hill Conglomerate - Flat Black Clasts Squared, Jagged
Edge Visible on Top Piece
Mars Hill has a different story.  The rocks that comprise this conglomerate are flat and black.  The flatness (and perhaps the darkness) suggests that this rock may be a shale formed in a  low energy environment.  This rock probably sat at the bottom of some ocean.  Layer upon layer of fine-grained sediment coasted to the seafloor at a ridiculously slow pace, with added phytoplankton and low oxygen levels coloring the sediment black.  Here too the land becomes elevated.  Streams shatter the shale and push it downstream.  The current softens the edges a bit before the slivers of rock reach an endpoint.

The endpoint, however is just an endpoint, not the end.  The beach, the ocean bottom, they are just the first step in the stories told by these rocks.  Check back soon for chapter 2 and 3.

Dietrich, Richard Vincent, and Brian J. Skinner. Rocks and Rock Minerals. New York: Wiley, 1979. Print.

Way, Bryan. "Black Shales." 1 Dec. 2006. Web. 28 June 2013. <>.

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