geochronometer
/ˌdʒiːə(ʊ)krəˈnɒmɪtə/Geology rare before mid 20th cent
noun
A feature, phenomenon, organism, etc., used as a means of dating geological events or (now historical ) ascertaining the age of the earth.
Origin
Late 19th century; earliest use found in The English Mechanic. From geo- + chronometer.