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By sea and rivers, North Wales

Fast-flowing streams in highlands like Snowdonia move rock grains, pebbles, and even large boulders for short distances, after heavy rain. During their long journey down river, the rock fragments may settle out and get picked up again many times, and become gradually more worn-down and smaller in size each time they are moved on.

When rivers meet the sea they often form estuaries like this one. As the current slows, sand and gravel is deposited to form sandbanks like those in the photo. The sandbanks are also re-arranged by tidal flows near the river mouth. The finest material, mostly mud, may settle out to form mudflats or be carried further out to sea where it finally settles out in calmer water.

Mawddach estuary, North Wales

Mawddach estuary, North Wales
Bottom curves