An ophiolitic suite is found in the Nicoya Complex, which is made up of cherts, graywackes, tholeiitic pillow lavas and basaltic agglomerates. It is intruded by gabbroic, diabasic, and dioritic rocks. The Cretaceous-Tertiary basin is made up of five formations of mainly marine origin. The Rivas and Brito formations are uplifted to the southeast and are overlain in the northwest by a slightly tilted marine near- shore sequence, the El Fraile formation. This in turn passes north into the undeformed Tamarindo formation, a sequence of shallow marine, lacustrine and terrestrial sediments interspersed with ignimbrites. Northeast of the Nicaraguan Depression, the Coyol and Matagalpa formations, run from Honduras to Costa Rica and still show evidence of some volcanic centres, distinguishable as constructional landforms.