start of a period, it is the practise to drive a gold spike into an internationally agreed level, thus formally establishing a single place where a period begins. However, the location of these spikes can be ill-informed, especially in the case of the basal Cambrian. Their limited geographical extent restricts their utility on other continents; a good stratum in each geological region is required to allow accurate cross-continental correlation. Even biostratigraphic markers fall foul of bioprovinciality, and are of limited utility in correlating rocks on opposite sides of the globe. Because a species may not appear in all habitats across the globe at the same time, defining the base of the Cambrian on the basis of the first appearance of one species is not desirable. See also — era the Cambrian period is under. References External links Geol.umd.edu: The Cambrian Period The Global boundary Stratotype Section and Point (GSSP) of the Drumian Stage (Cambrian) in the Drum Mountains, Utah, USA