I never get tired of quoting that guy in the Times who wrote succinctly that “Manhattan is gneiss, but full of schist.” The bare bones of the little island of Mannahatta are exposed on the upper, upper west side where a ridge of mica schist, the famed Manhattan schist, rises over the flatlands of Harlem. The pictured flank is in St. Nicholas Park, at 137th and Saint Nicholas Ave. The neo-Gothic buildings of CCNY loom above.
That sure looks like rock varnish to me, something more famous in the southwest. Also going on here is sheeting, or contour weathering, a type of rock exfoliation due to the chisel of time and weather.
And in a little hollow, a patch of Mugwort (Artemisia)! The stuff grows anywhere. Its roots, like other plants punching into rock, will assist in the weathering away of this rock face over time.
Backyard and Beyond 
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