Tuesday, August 19, 2014

Skipping Stones at Kettle Cove

An assortment of stones at Kettle Cove,
 including white quartz and gray phyllite 
Chasing my fifteen month old son at Kettle Cove beach means faster glances at the rocks. I absorb the scene but miss a lot of the details.  A lot of flat rocks - good for skipping.  "That's a little too deep bud, let's bring it in." Some nice round hunks of quartz. "It's okay, keep walking and you'll dry off ." More of the same; more flat, more quartz. Not a lot to see, but that says a lot.

Arms of dark rock contain the sands of Kettle Cove beach
Arms of dark rock supply the cove of Kettle Cove. The repeating foliations and large "knots" make it look almost like wood.  A fact that drives the untutored to ask whether it is petrified wood.  It is not.  The physical features have a more involved backstory.  Four hundred twenty million years ago this rock was sediment in a deep ocean.  Sand, silt and clay piled up in a space between continents: to the north what would one day become most of North America to the south Avalon, a microcontinent whose remains form the coast of Maine and parts of Rhode Island, Great Britain and Africa.  It might seem that these layers of rock would become the "tree rings", but the thin sheets would reveal themselves millions of years later.

"Petrified wood" is really ocean bottom rock
split by stretched bands of quartz 
Four hundred million years ago Avalon crept toward  the North American core.  The pressure on these layers escalated. The minerals in mud migrated and aligned themselves to sustain the stress.  The wavy foliations are the result.  The heat and pressure of impact had the added effect of heating quartz to liquid, sending it screaming through cracks and layers.  As things settled down the quartz solidified into extended blocks called dikes.

The collision with Avalon was prelude to the formation of Pangea. As this is now the coast, we know these impacts were not the end of the story. Africa bumped our coast, and then bounced back out to sea (carrying a piece of Avalon with it). Our "petrified wood" does not have stripes of quartz.  It is punctuated by "knots" of it.  As the continents split, the bands of quartz stretched. Some parts spaghettified into ribbons.  Other sections remained thick with tapered edges.  These bulbs, called boudins (French for sausage, which the strands of quartz resemble) become the "knots" in our wood grain.

Inevitably the rock of this formation, having been lain down, rearranged, injected with quartz and stretched, began to break down. Glaciers, rivers and waves have all had their shot at the rocks of the Maine coast, and the most recent of these don't carry rocks very far.  The smallest pieces may end up as mud on the ocean bottom or sand on the beach. The larger chunks stay local. The flattened layers become skipping stones. The round quartz knots roll back and forth in the surf. These remnants remain for my son and me to explore while perusing Kettle Cove.

Bentley, Callan. ""Boudinage" is my favorite geology word - Mountain Beltway - AGU Blogosphere." Mountain Beltway Site Wide Activity RSS. American Geophysical Union, 20 June 2011. Web. 19 Aug. 2014. <http://blogs.agu.org/mountainbeltway/2011/06/20/boudinage-favorite-geoword/>. 

Berry , Henry N. , and Robert G. Marvinney. "The Geology of Two Lights State Park Cape Elizabeth, Maine." Geologic Site of the Month. Maine Geological Survey, n.d. Web. 19 Aug. 2014. <http://www.maine.gov/dacf/mgs/explore/bedrock/sites/jun02.pdf>.