This paper describes work extending Princeton WordNet to the domain of geological texts, associated with the time periods of the geological eras of the Earth History. We intend this extension to be considered as an example for any other domain extension that we might want to pursue. To provide this extension, we first produce a textual version of Princeton WordNet. Then we map a fragment of the International Commission on Stratigraphy (ICS) ontologies to WordNet and create the appropriate new synsets. We check the extended ontology on a small corpus of sentences from Gas and Oil technical reports and realize that more work needs to be done, as we need new words, new senses and new compounds in our extended WordNet.