Petroglyphic rock art in three valleys of Mongolia's Altai Mountains reveals the anatomy of deep time at the boundary between Central and North Asia. Inscribed over a period of twelve millennia, its subject matter, styles, and manner of execution reflect the constraints of changing geology, climate, and vegetation. These valleys were created and shaped by ancient glaciers. Analysis of their physical environment, projected from the deep past to the present, begins to explain the rhythm of cultural manifestations: where rock art appears, when it disappears, and why. The material and this remote arena offer an ideal laboratory to study the intersection of prehistoric culture and paleoenvironment.