Tianzhu Peak, the main peak of Tianzhushan, so named for its resemblance to a towering pillar, is a bornhardt or mountain-island which is composed of medium-grained monzonitic granite from the early Cretaceous period (128 million years ago). Since the vertical and diagonal joints are well-developed and intersect with each other on granite, deep weathering occurs, which causes the disintegration and collapse of the rock mass, eventually forming the pillar-like peak. The peak is erect in the center but slightly outward-inclined on the periphery.