The gradual rounding of the corners and edges of angular blocks of rock is called exfoliation mechanical weathering spheroidal weathering unloading

Answer is: spheroidal weathering.
Spheroidal weathering is a form of chemical weathering that and results in the formation of concentric layers of rock. 
Spheroidal weathering causes rocks to decompose at their edges, allowing water to reach the surfaces.
Spheroidal weathering is common among granite and similar rock types.

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