The geology of the park consists of metamorphosed sedimentary sequence of Palaeocene-Miocene age. Mount Mulu is located at the southwest of the park. It was formed from Palaeocene-Upper Eocene with the interbedding of sandstone and shales. The remaining mountains (Mount Api, Mount Benarat, and Mount Buda) are located the western flank of Mount Mulu. These three younger mountains were formed from 40 km band of karstic limestones during the Upper Eocene-Lower Miocene period. It is in this band that the extensive cave system was developed. The cave formation was the result of tectonic uplift of the karst at 2 to 5 million years ago. The caves are typical of the tropical river caves, which exhibits various features such as: elliptical tubes linking different levels of the cave, and speleothem including aragonite and calcite needles. The limestone are either extremely white or grey. The pinnacles on Mount Api are the result of extreme weathering of limestone rocks.