Rill marks are distinctive patterns formed on sandy beaches as water flows downhill after the tide has fallen.
As the tide rises over a sandy beach, the gaps between the grains of sand fill with water.
When the tide starts to fall, this water is pulled down by gravity. Much of the surface water washes over the top of the sand and returns to the sea, but some water remains deep in the sand itself.
Gravity pulls this water vertically downwards, not towards the sea. As this water travels down it meets a layer of impervious clay or stone. This water then starts to flow downhill over this barrier until it emerges further down the beach.
As the water trickles out over the sand it creates channels. They are distinctive and ‘dendritic’ – they look a little like trees. These channels are called ‘rill marks’.
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