There are many time scales, some of which are provincial and(or) highly specialized (e.g., North American land mammal ages). Therefore, it's especially important to cite in your publication the time scale you're using. Here, we provide three that are significant and widely used:

• USGS Geologic Names Committee (earlier versions -- 2018   -  2010   -  2007)
• International Commission on Stratigraphy
• Geological Society of America, v.6.0

Geolex's content is drawn from the literature published since the late 1800's. Throughout that time, scientific knowledge has increased, thereby leading to modifications to the geologic time scale. Because the definitions of geologic time intervals (e.g., the Ordovician) have been modified as more information is gathered, the geologic age of a unit as stated in a report published in, for example, 1950, may be different according to today's time scale. It is critical to be aware of this when studying older reports.

In Geolex, the originally-reported ages as shown in the summary paragraphs on the "Significant Publications" pages are being "migrated" to the modern time scale, to be shown on the "Unit Summary" pages. This is an arduous task, especially when considering that during the past 30 years, portions of the geologic time scale have been subject to extensive and frequent revisions.