 Roughly 60 to 90 million years ago, the outpouring of basic lava through fissures formed horizontally bedded basalt over large areas.Variations in their composition and structure have resulted in massive, well-jointed steel-grey cliff faces alternating with structural benches of vesicular amygdaloid lava and ash layers, all of which contribute to the pyramida-shaped hills and crest-level plateaux or mesas.Earth sculpturing under the tropical climate completed the panorama-sharply defining the landform features in the semi-arid conditions, and rounding the hilltops under water condition.