Chocolate Hills National Geological Monument

October 1, 2010

Chocolate Hills National Geological Monument

(Below is what was written in the plaque on the viewdeck at the Chocolate Hills Complex, Carmen, Bohol)

1 About two million years ago, most of the island of Bohol was below a shallow sea. Coral reefs, similar to those now found offshore of northern Bohol, thrived and extensively covered the sea floor. During stormy days, fragments of corals and shells derived by waves from the reefs were deposited mostly at the landward side of the reefs. The coral and shell fragments formed relatively thin layers (brown) surrounding the live coral reefs (pink).

2 Slowly, the land rose causing the coral reef formations to emerge out of the sea. The taller arrows shown in the diagram indicate that the southern sections of Bohol island had been uplifted more than the northern sections. 

3 The chocolate hills had been carved out from the relatively thin layers of coral and shell fragments. During the initial stage, the surface may have looked like as shown in the diagram which covers the brown rectangular patch in the previous diagram. Gullies were developed in the low lying areas and lakes occupied the existing depressions.

4 Coral and shell fragments are largely composed of calcium carbonate. A chemical compound which can be dissolved by acidic solution. Rainwater becomes slightly acidic by dissolving some carbon dioxide from the atmosphere. Thus, rainwater can dissolve coral and shell fragments. But this happens very slowly. With rainwater acting on the layer of coral and shell fragments for tens to hundred of thousand of years, gullies were deepened and widened to become streams. Lakes were emptied by underground rivers and interconnected conical hills were formed from the original flat surface.

5 When the base of the soluble formation was reached, downcutting ceased and later erosion became dominant. Valleys were widened. The remnant of the layers in the highly dissected areas were dissolved away. And the connections between neighboring hills were also dissolved away. Thus, the chocolate hills that you now behold are products of the patient laboring of rainwater on a thin soluble limestone formation.

6 Based on geological studies conducted by the Mines and Geosciences Bureau of the Department of Environment and Natural Resources, there were parts of Bohol that used to be under the sea. These are shows as the blue colored areas in the map. The parts above sea level, which were confined mostly in the north, northeaster and eastern sections, are color-coded green.

The location of the area covered in the first and second block diagrams shown on the left is the rectangle outlined in the map.

7 Declaration

The unique landform known as the "Chocolate Hills" of Bohol was formed ages ago by the uplift of coral deposits and the action of rainwater and erosion.

These hills are scattered throughout the towns of Carmen, Batuan and Sagbayan and consist of 1,268 mounds of the same general shape. 

In recognition of its special characteristics, scientific importance, uniqueness and high scenic value, the national committee on geological sciences declares the chocolate hills of Bohol a National Geological Monument.

19 June 1988


Department of Environmental and Natural Resources

General Manager
Philippine Tourism Authority

National Committee on Geological Sciences

Sub-Committee on Geological Monuments