The park lies on a plain, the ancient Nicaraguan Trench, which was filled with sediments 7 million years ago. Small ridges loom here and there, such as Lomas de Sierpe (311 metres), which are ancient volcanic cones composed of lava, pyroclasts, andesite and pumic stone. The rivers meander through this region carrying silt, which the coastal tides shift in parallel strips, giving rise to long straight beaches. These shores of dark sands, pounded by the wild surf, are the ideal place for sea turtles to lay their eggs. Tortuguero is used by the hawskbill, Pacific green, a species which measures 1 metre long ond weighs up to 200 kilos, and the leatherback, the largest sea turtle in the world , weighing up to 700 kilos.
The Pacific green turtle nests from July to October, the leatherbacks from February to July, with greater frequency during the months of April and May, and the green turtles from July to October, with greater activity in August. Another hatchery for these species is the neighbouring Barra de Matina south of the park.