An uplifted coral reef
The Chocolate Hills are an example of conical karst topography. Their story begins during the Late Pliocene to Early Pleistocene epochs, when the area of Bohol island was a shallow sea. Marine organisms like corals, foraminifera, and mollusks thrived. As they died, their calcium carbonate remains accumulated on the seafloor, forming thick layers of marine limestone.
Tectonic forces, driven by the collision of the Philippine Sea and Eurasian plates, caused the land to rise. This uplift exposed the limestone beds to the atmosphere for the first time. What was once a bright reef became a plateau of soft, porous, fossil-rich limestone resting on a layer of hardened clay. This geological stage prepared the area's transformation.
Sculpted by rainwater
Once exposed, the limestone began to weather. Rainwater, made slightly acidic by absorbing atmospheric carbon dioxide, seeped into the rock's fractures and joints. This acidic water slowly dissolved the calcium carbonate in the limestone, a chemical weathering process known as karstification. Over thousands of years, surface water and groundwater carved away at the soluble rock, forming a network of caves and springs between the hills.
The erosion left behind the more resistant parts of the limestone as thousands of nearly symmetrical cones. Today, there are at least 1,260 hills, with some estimates as high as 1,776. They are spread over an area of more than 50 square kilometers (20 sq mi). The hills typically range from 30 to 50 meters (98 to 164 ft) in height, though the largest reaches about 120 meters (390 ft). During the dry season, the grass covering the thin topsoil turns brown, giving the area its name. The hills were declared the Philippines' third National Geological Monument on June 18, 1988.