A GRAPHICAL REPRESENTATION OF THE GEOLOGIC TIME CAPTURED IN THE WORLD HERITAGE

A total of 96 geoheritage sites are currently inscribed on the UNESCO World Heritage List because of their Outstanding Universal Value. The sites are grouped in 11 themes corresponding to the dominant geologic or geomorphological process, and each site covers one or more divisions of geologic time based on the age of its fossils or rocks, its main process, a prominent feature or a memorable event.

The data was collected from the site descriptions available on the UNESCO website and the UNEP-WCMC datasheets, and was complemented with various sources of literature. For visual purposes, geologic time was represented at different scales: (1) the Precambrian supereon and the Phanerozoic eon, (2) the Phanerozoic eon only, and (3) the Quaternary period. Themes were plotted individually and collectively. A series of stacked bar charts and a color gradient were used to depict the age range of each site and the age frequency (i.e. how many sites are of the same age).

Overall, half of geologic time is represented, with some divisions being overrepresented (e.g. the Phanerozoic eon, the Quaternary period), while others being underrepresented (e.g. the Precambrian supereon, the Proterozoic eon). The representation of the Archean eon as a continuous range based on poorly-constrained ages from one site is likely inaccurate. The representation of the Proterozoic eon as discrete pinpoints based on well-constrained ages from multiple sites is more accurate.

The Phanerozoic eon has a continuous representation within fossil sites, fluvial, karst, glacial and desert systems. The best represented periods are the Ordovician-Silurian, the Permian-Triassic and the Cretaceous. The fossil record is almost uninterrupted throughout the Cenozoic and the Mesozoic, and becomes scarce in the Paleozoic.

The Quaternary period is also captured continuously, with the Pliocene, Pleistocene and Holocene being overwhelmingly represented, especially by sites of volcanic, karst and glacial origin.

The representation of geologic time declines from the present to the past. This pattern reflects the very nature of Earth's evolution, whereby younger features and processes are invariably better preserved than features and processes of the distant past, which have been eroded or buried. The pattern is hypothesized to remain unchanged irrespective of the number and nature of future site designations.