Tobago, an island of the British West Indies, lies 20 miles north of Trinidad. The oldest rocks exposed are isoclinally folded schists and phyllites, composed largely of metavolcanic material, outcropping in the mountainous northern part of the island. South of the schist is a belt of strongly sheared but relatively unaltered igneous rocks, including ultramafic and dioritic intrusives, and andesitic and basaltic volcanics. Flat-lying, fossiliferous upper Miocene-Pliocene sands and clays rest unconformably on the volcanics near the southwest end of the island, and coral limestone overlaps both igneous rocks and upper Tertiary sediments.
A review of West Indian geologic history suggests that the Tobago schists are mainly Cretaceous, that the schist-forming movements came in late Cretaceous time, and that a second period of diastrophism, resulting in the shearing of the younger igneous rocks, occurred in the middle or late Eocene. Igneous activity apparently ceased prior to the second diastrophism.
Except for flooding of its margins, Tobago has probably been a land area since late Cretaceous time. During the late Cenozoic, marine sediments accumulated along the present coast line and in drowned stream valleys, subsequently re-excavated. An embayed shore line and prominent sea cliffs indicate recent relative submergence of the island.