Rock varnish commonly occurs in arid environments. It can coat not only rock surfaces but also any exposed prehistoric stone artifacts and rock art. Thin sections of varnish coatings reveal microlaminations that reflect major changes between wet versus dry paleoclimates. These microlaminations can be used to assign minimum ages to the underlying rock surface, providing dates for otherwise undateable stone artifacts and rock art. This dating approach has provided important information about the peopling of the Americas (North and South America), including how and when the native American populations adapted to changing climatic conditions.