Actually granite is not hard as it has fractures.

When the rocks on either side of the fissure do not move along the fracture surface, the fracture is called a joint. Rocks may be weathered along the joints. This is an important factor that affects the weathering of rock.

 Classification of joints

Joints that formed along with the formation of the rock are known as “primary joints”; those resulting from rock movement or weathering are called "secondary joints".

There are three sets of primary joints in granite: flat-lying joints, vertical joints and diagonal joints.

Three sets of primary joints in the Tianzhushan

In the formation of granite landform, they are the vertical joints but not diagonal joints that usually form steep mountains and peaks. And flat-lying joints create gentle hills.

Vertical joints control the formation of peak clusters.