Rock varnish, also known as desert varnish, is a thin layer on a rock surface, typically composed of clay minerals, manganese and iron oxides, and trace amounts of over 30 minor compounds. The use of rock varnish as a dating tool by two methods is discussed: the change in the ratio of potassium and calcium relative to titanium which declines with time; and radiocarbon dating using accelerator mass spectrometry. The possibility of dating pictographs, petroglyphs, and geoglyphs (stone alignments on the ground) is suggested. (Abstractor's note: in <i>American Scientist</i> Vol. 80, No. 2, pp. 110-113, (Mar-Apr 1992), Paul Bierman et al., in a letter to the editor, suggest that this article presents an incomplete review of rock varnish research and that rock varnish dating techniques are not widely accepted and reliable.)