Puerto Rico is composed of Cretaceous to Eocene volcanic and plutonic rocks, overlain by younger Oligocene and more recent carbonates and other sedimentary rocks. Most of the caverns and karst topography on the island occurs in the northern region in the carbonates. The oldest rocks are approximately years old (Jurassic) and are located at Sierra Bermeja in the southwest part of the island. They may represent part of the oceanic crust and are believed to come from the Pacific Ocean realm.