Nicolaas Witsen (1641-1717) is known not only as an influential administrator and diplomat, but also as a researcher, collector and author with very wide-ranging interests. His best-known publications are Architectura navalis et regimen nauticum, which soon became a standard work on shipbuilding, and Noord en Oost Tartarye, in which Siberia and the surrounding areas are described from a variety of perspectives. This article highlights an aspect of Witsen’s work that has remained unstudied so far: his geological and archaeological observations in Amsterdam and the surrounding area of Amstelland, recorded in the manuscript Natuer van de gront rontsom Amsterdam. This is interesting not only because of Witsen’s observations in and around Amsterdam, but also from a scientific-historical perspective in that Witsen’s manuscript contains insights that place its author at the birth of modern geology.