The rocks that form much of the Santa Elena peninsula in the extreme western sector of the park are among the oldest in Costa Rica dating back to the Cretaceous period, some 130 million years ago. The forested portions of the park comprise a large percentage of the precious little remaining area of tropical dry forest habitat throughout all of Central America.

In addition to providing refuge to the many plants and animals adapted to dry forest conditions, Santa Rosa protects two important sea turtle nesting beaches, Naranjo and Nancite. The latter is one of two beaches in Costa Rica (the other is Ostional) where Pacific Ridley Sea Turtles come ashore each year in large concentrations to lay their eggs.

Among the many creatures inhabiting the 49,515 hectares of Santa Rosa National Park, some of the more commonly seen species include: White-tailed Deer, Long-tongued Bats (that roost in the historic buildings), White-throated Capuchin Monkeys, White-throated Magpie-Jays, Orange-fronted Parakeets, and Roadside Hawks.

The vegetation in most parts of the park is characterized by being deciduous and many plant species remain leafless for much of the dry season as a means of conserving precious moisture. Another common feature of the dry forest vegetation is the high percentage of plants that have thorns or spines. The prevalence of this form of mechanical protection has led tropical biologist, Dr. Daniel Janzen, to speculate that this defense evolved eons ago in response to the pressure from huge herbivores such as Giant Ground Sloths and Mastodons that once roamed this region. Whatever the evolutionary reason may be, do be careful what you touch or brush up against in the tropical dry forest!