When the of the sea or waves is high (with big waves and rough seas), boulders and large pebbles can be moved. When the is low (with small waves), only sand can be moved.
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Waves have the power to erode, or break up, and wear away rocks over time. Storm, or high-energy, waves breaking at the foot of a cliff dislodge blocks of stone - rocks. Waves fling pebbles at the foot of a cliff and wear the cliff away. This process is abrasion.
The broken rocks get moved around on the beach and are rounded and worn down smaller.
of the sea or waves is high (with big waves and rough seas), boulders and large pebbles can be moved. When the 
