Global Boundary Stratotype Section and Point

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A GSSP — the "golden spike" — is a physical point in a physical outcrop that defines the base of a geological stage for the whole planet. The distinction that matters is between definition and measurement: the boundary IS the marker horizon in that section, usually the first appearance of a chosen fossil species, and the numerical age attached to it is a separate, revisable measurement of that same horizon. This is why a boundary can be redated without being moved. It also explains an oddity readers often trip on — the base of the Cambrian is defined by the first appearance of a burrow, the trace fossil Treptichnus pedum, rather than by any of the animals the Cambrian is famous for, because a definition must be recognisable in ordinary rock rather than only in exceptional deposits. GSSPs are ratified by the International Commission on Stratigraphy and subsequently by the International Union of Geological Sciences; the majority of Phanerozoic stages now have one, while Precambrian boundaries are still mostly defined by agreed numerical ages instead.

A node is not a place. Drawing it on a map would assert something about the world that no stored fact supports.