The gradual rounding of the corners and edges of angular blocks of rock is called

Weathering

Explanation:

In the process of weathering, rocks break down into smaller pieces. The corners and edges of the rocks are shaped by the weathering process to form gradual rounded rock pieces.

Weathering causes "spalling off" of  the rock joints, layer by layer to produce rounded /dome boulders or rocks such as granite and ash tuff.  

For example - Exfoliation is a form of mechanical weathering which breaks the cruved plates of rocks to produce rounded rocks.

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