Answer
The process that transforms angular blocks of bedrock into rounded boulders is known as abrasion. Abrasion occurs when rocks and sediments are transported by wind, water, or ice, and their surfaces come into contact with each other or with the surfaces of the landscape.
As these rocks and sediments are transported, they collide and rub against one another, causing the edges and corners of the angular blocks to be worn down and smoothed over time. The repeated impact and friction between the rocks lead to the removal of small fragments and the gradual rounding of the boulders.
The specific mechanism of abrasion depends on the medium of transport. In rivers and streams, for example, flowing water carries sediments downstream, causing them to collide and rub against each other as well as the riverbed and banks. Glaciers also contribute to abrasion by grinding rocks against the underlying bedrock as they move. Wind-driven sand or ice crystals can also abrade rocks through constant bombardment.
The size and shape of the resulting boulders depend on factors such as the hardness of the rocks, the duration of transport, and the intensity of the abrasion process. Over time, the angular blocks of bedrock gradually become more rounded as their rough edges and corners are worn away by the abrasive forces, resulting in the formation of smooth, rounded boulders.
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