Silurian graptolites and Ordovician and Silurian chitinozoans have been found in well cores from the argillitic basement rocks of the Prudhoe Bay oil field and Naval Petroleum Reserve No. 4 on the North Slope of Alaska. Monograptus spiralis (Geinitz), a late Llandoverian graptolite, and Middle Ordovician to Silurian chitinozoans belonging to the genus Conochitina Eisenack were discovered at a depth of 12,937-12,938 ft (3,943.2-3,943.5 m) in the ARCO-Humble Sag River State 1 well at Prudhoe Bay. Silurian chitinozoans of the genus Bursachitina Taugourdeau were found at a depth of 2,869-2,879 ft (874.5-877.5 m) in South Barrow test well 3 (NPR 4). South Barrow test well 1 yielded Silurian chitinozoans of the genus Sphaerochitina Eisenack at a depth of 3,341-3,346 ft (1,018.3-1,019.9 m) and the species Conochitina micracantha Eisenack, Cyathochitina campanulaeformis (Eisenack), and Lagenochitina baltica Eisenack, indicative of a Caradocian through Ashgillian age, at a depth of 3,535-3,537 ft (1,077.5-1,078.1 m). These findings prove that the basement rocks of the Barrow arch are of approximately the same age, at least over the 200 mi (325 km) between Prudhoe Bay and Point Barrow.