In 1973 Robert Smithson made a series of works he titled Slate Grinds. Each comprises a section of slate on which he incised a partial circle. These late works by Smithson encompass a number of his central interests: materiality, deep time, aerial views, a critique of extractive industries, and geological history. Through the Slate Grinds Smithson forges a direct connection with geologic time, mark-making through the geologic process of erosion.

The first of Smithson’s major outdoor earthworks intended to exist exclusively outside, ​Asphalt Rundown​ is a demonstration of what Smithson called the “crystalline structure of time.” He argued that time does not pass so much as it builds upon itself. In a quarry outside of Rome, Smithson poured a truckload of hot asphalt down a steep embankment, which cooled and hardened as it fell. The resulting sculpture can be seen as time frozen, mid-flow, or as yet another sedimentary layer in the infinite accumulation of time.