Tortuguero National 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/1,020 ft), which are ancient volcanic cones composed of lava, pyroclasts, andesite and pumice 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 hawksbill turtle, Pacific green turtle – a species that measures 1 metre (3 ft) long and weighs up to 200 kg (440 pounds), and the leatherback, the largest sea turtle in the world. The wildlife is rich and diverse. Monkeys, anurans (of which 60 species have been observed), birds (of which 309 species have been identified) and fish are especially numerous. The Tortuguero wilderness areas offer some of the country’s greatest biological variety. The total number of habitats identified in the park is 11, including littoral woodland, high rainforest, slope forest, swamp forest, holillo forest, herbaceous swamp and herbaceous lagoon communities.