Purplehorse Beach

Couple looks at map while resting on the rocks of Purplehorse Beach

NPS / Nanette Nyce

THE FORCE SPECTRUM - Purplehorse Beach

This beach found along Billy Goat Trail A contains rocks that are not absolutely metamorphic or igneous, called “mixed rocks” or migmatites. Sometimes, when rock composed of various minerals with different melting temperatures is exposed to subsurface tectonic heat and pressure, some minerals melt & become igneous while others only metamorphose.

Migmatites are like chocolate chip cookies. The igneous rocks are the chocolate chips and the the metamorphic is the cookie portion. Like chocolate chip cookies, there are visible boundaries between the igneous rock and the metamorphic. It’s possible that these rocks were formed in the heated, pressurized core of a the Appalachian mountain belt.

 

Migmatite banding on outcrop with water bottle for scale.

NPS /  Nanette Nyce

A band of migmatite crosses the C&O Canal from about MM 12.5 - MM 16. In addition to Purplehorse beach, migmatized metagraywacke can be seen next to the towpath along Widewater (MM 12.6).

In nature, multiple forces work in conjunction, forming a spectrum of effects. Investigating natural causes can be complicated and requires continuous study and research.

Discussion Question: What are some natural (and humanmade) forces that work together to cause a spectrum of effects, altering the environment?

Additional C&O Geology pages: Locks 15-20 | Olmsted Island | Flooding | Devils Eyebrow | Pothole Alley | Widewater | Lamprophyre Dikes | Mather Gorge | Paw Paw Tunnel | Cumberland Coal

Return to main Rock Talk page.

 

Close up of migmatized metagraywacke along Widewater.

NPS / Nanette Nyce