Geologically, the islands are not part of the Lesser Antilles Volcanic Arc. Rather, Trinidad was once part of the South American mainland and is situated on its continental shelf, and Tobago is part of a sunken island arc chain related to the Pacific-derived Caribbean Plate. The islands are separated from the continent of South America by the Gulf of Paria; Bocas del Dragón, a 19-kilometre (11.8 mi)-wide northern passage; and Serpent's Mouth, a 14-kilometre (8.7 mi)-wide southern passage.
The Northern Range consists mainly of Upper Jurassic and Cretaceous metamorphic rocks. The Northern Lowlands (East–West Corridor and Caroni Plains) consist of younger shallow marine clastic sediments. South of this, the Central Range fold and thrust belt consists of Cretaceous and Eocene sedimentary rocks, with Miocene formations along the southern and eastern flanks.The Naparima Plains and the Nariva Swamp form the southern shoulder of this uplift.
The Southern Lowlands consist of Miocene and Pliocene sands, clays, and gravels.These overlie oil and natural gas deposits, especially north of the Los Bajos Fault. The Southern Range forms the third anticlinal uplift. It consists of several chains of hills, most famous being the Trinity Hills. The rocks consist of sandstones, shales, siltstones and clays formed in the Miocene and uplifted in the Pleistocene. Oil sands and mud volcanoes are especially common in this area.
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