Correlations Build the Relative Time Scale

Changjieng in China and the Burgess Shale in Canada's Yoho National Park share a similar fauna, but the Chinese site (right) was deposited in early Cambrian time, the Burgess Shale (left) in middle Cambrian time.

Paleontologists and geologists first worked out a relative timescale from organisms found in sedimentary rocks, cross-cutting relationships in the rocks and geologically instantaneous events like volcanic ashfalls, turbidity flows and floods.

Igneous rocks may give "absolute" dates where they bracket or cross-cut Neoproterozoic and Cambrian strata.

Both photos from National Geographic, October, 1993