Human footprints & animal - Indian petroglyphs (~100 B.C. to ~1540 A.D.) on desert varnish-covered quartzose sandstone (Wingate Sandstone, Lower Jurassic) in Utah.

This is part of a fantastic panel of prehistoric and historic petroglyphs that was carved by several groups - Archaic Indians, the Anasazi, Fremont, Navajo, Pueblo, and Ute. The dark coloration of the rock is desert varnish - a mineral-encrusted rock surface that is common in many desert settings. Desert varnish is composed of clay minerals, brick red-colored hematite, and/or black-colored manganese oxide minerals (for example, birnessite - www.mindat.org/min-680.html).

Scraping or hammering away at the desert varnish exposes the light-colored sandstone. The oldest petroglyphs are identified by the partial development of another layer of desert varnish - they are dark-colored than younger petroglyphs.