|Desert Varnish is a dark coating found on
in arid environments.
Varnish can be a prominent feature in landscapes in northern Arizona. It
often coats canyon walls, particularly in the areas where water flows
down the the sides of canyons.
Native Americans created
petroglyphs by scraping or chipping away the dark varnish to
expose the lighter rock beneath.
coating is composed dominantly of fine-grained clay minerals that
attach to the wall via airborne dust. In the clay layer, black
manganese oxide (the mineral birnesite) and red iron oxide (the
mineral hematite) add color.