Navarino Island is composed mainly of a sequence of geosynclinal sediments—the Yahgan Formation—at least 3,000 metres thick and of probable Early Cretaceous age. The main sediments are greywackes, argillites, and cherts. Calcareous concretions are characteristic of some horizons but are nowhere abundant. Many coarser-grained sediments contain abundant andesitic to basaltic debris, indicating widespread contemporaneous volcanic activity. Fossils are rare and comprise poorly preserved Radiolaria, fragmentary plant remains, and imprints ofInoceramus. Recrystallisation is evident in many rocks; in particular, secondary prehnite is characteristic, especially in the coarser sediments. The formation includes several thick quartz dolerite sills. The largest of these forms a conspicuous east-west chain of serrated peaks, the so-called Dientes de Navarino. The south-west part of the island and the nearby Bertrand Island are composed of altered basic igneous rocks, believed to be contemporaneous with the Yahgan Formation sediments.