Beachrock

On my walk along the shore today I was taken by this image of beachrock (pebbles and sand, compressed over the centuries to form a sort of natural concrete).... I love the texture (notice the brown line across the image, this is the iron-pan - the level at which iron is leached out of the soil and re-deposited as it crystallises in the earth, forming an almost unbreakable layer of naturally-occurring iron)