The Chocolate Hills of Bohol in the Philippines present one of the world’s most visually striking landscapes. Clustered across the island’s interior, these formations are remarkable for their conical shape and sheer abundance. They are covered in green grass most of the year, but during the dry season, this vegetation turns a rich cocoa-brown, giving the hills their famous name. The unusual symmetry and distribution of these mounds have long inspired questions regarding their origin.
Defining the Landscape
The Chocolate Hills are a collection of conical mounds clustered across the municipalities of Carmen, Batuan, and Sagbayan in Bohol Province. The total count is estimated to be between 1,260 and 1,776, spreading over 50 square kilometers. The hills are symmetrical and haycock-shaped, varying in height from 30 to 50 meters, with the tallest reaching approximately 120 meters.
These distinctive mounds are composed primarily of marine limestone, characterizing the region’s underlying geology. The landscape is a textbook example of cockpit karst terrain, a topography formed by the dissolution of soluble rocks. The name “Chocolate Hills” is derived from the seasonal color change that occurs when the thin layer of grass covering them dries out, turning from vibrant green to chocolate brown.
The Accepted Geological Process
The explanation for the Chocolate Hills involves marine deposition, tectonic uplift, and chemical weathering. The process began in the Late Pliocene to Early Pleistocene epochs when the area was submerged beneath the ocean. Skeletal remains of shallow marine organisms accumulated on the seabed, forming thick layers of calcium carbonate that solidified into soft, fine-grained marine limestone.
Tectonic forces caused the seafloor to rise. This process of orogenic uplift elevated the limestone beds above sea level, exposing the ancient marine sediments to the elements. Once exposed, the uplifted limestone platform became susceptible to karst dissolution.
The conical shape resulted directly from chemical weathering by tropical rainwater. As rain falls, it absorbs carbon dioxide, creating a weak carbonic acid solution. This slightly acidic water dissolves the soluble limestone along natural fractures and fissures in the rock. This continuous dissolution, known as crypto-corrosion, sculpted the uplifted plateau into individual, rounded mounds.
The specific cockpit or haycock karst shape is attributed to a porous limestone base and a consistent, high-humidity, monsoonal climate. The uniform distribution of rainfall prevents one side of the hill from eroding faster than the other, resulting in the strikingly symmetrical slopes.
Unlike other karst regions, the Chocolate Hills area is characterized by these residual limestone hills, or mogotes, separated by flat plains. This indicates a unique balance between uplift and erosion. The underlying hardened clay likely prevented the complete dissolution of the limestone, leaving the resistant conical remnants seen today.
Alternative Explanations and Mythology
Before detailed geological processes were understood, various hypotheses attempted to explain the hills’ origins. Local folklore provides a rich cultural context, often featuring tales of colossal beings. One legend tells of two feuding giants who hurled rocks at each other, creating the scattered mounds. Another myth describes a giant named Arogo whose tears, shed after his lover Aloya died, dried up to form the cone-shaped hills.
Early scientific theories suggested a volcanic origin or that the hills were limestone blocks created by faulting or tidal movements. These ideas were later dismissed because the rocks lack volcanic material, and the uniform shape is incompatible with simple tectonic faulting. Evidence of abundant marine fossils within the limestone firmly established the dissolution of uplifted marine sediments as the true formation mechanism.