How do I recognize geologic change in my environment? What is the tectonic journey of this rock?

Franciscan Mélange
Environment of formation:	oceanic trench near a subduction zone

Mélange isn’t really a type of rock. Rather, it is a mashed-up mixture of Franciscan rocks, mixed and crushed in the subduction zone. Dr. David Alt and Dr. Donald Hyndman, from the University of Montana, call mélange “a chaotic mess of rocks swept together within an ocean trench.” Sometimes the rock material in mélange is so sheared and crushed, it is hard for a geologist to identify what is there.

Mélange on Baker Beach near Fort Point containing graywacke, shale and serpentinite

Often, zones of mélange in the park are also areas where landslides occur.

Road in a Marin Headlands mélange zone being slowly destroyed as the land slides from under it

Mélange is a French word meaning “mixture.”

Rockslide containing blocks from a mélange zone at Black Sand Beach, Marin Headlands

How do I recognize geologic change in my environment? What is the tectonic journey of this rock?

Graywacke Sandstone
Classification: sedimentary
Environment of formation: near subduction zones

Graywacke sandstone at Baker Beach, San Francisco

Graywacke (GRAY-wak-ee) sandstone is a coarse-grained sedimentary rock that is mostly composed of quartz (silica), but also contains abundant feldspar, volcanic rock
fragments and silt. Since the sand and silt grains making up graywacke are all different sizes and shapes (poorly sorted), geologists call it a “dirty” sandstone. This sandstone appears dark gray, brown or green. It looks tan or rusty when weathered, because many of the rock fragments contain iron. The “poor sorting” is a clue that the material that formed this rock was transported and deposited rapidly. Graywacke sandstone forms near subduction zones where mountain-building provides much sediment and earthquakes produce underwater landslides that carry sand out into the deep ocean. Graywacke feels very gritty, like sandpaper.

The name, graywacke, comes from an 18th century German mining term, grauwacke, mean- ing “gray gravel.”

 	 	

Sandstone (at right) and shale (at left) in Crissy Field riprap

Tan sandstone with layers of dark shale at Baker Beach

Fossil ammonite from Baker Beach sandstone