stratigraphic intervals
   (System) †Ordovician (485.4 - 443.8 Ma)
     (Series) †Upper (458.4 - 443.8 Ma)
       (Stage) †Ashgillian
         1 (Substage) †Hirnantian
         2 (Substage) †Cautleyan + Rawtheyan
         3 (Substage) †Pushgillian