The natural order of things.

The tide is very low today. This is part of the small stone-built sea-wall that prevents erosion around the foundations of the road bridge over the river Ythan. 
The horizontal bands are an example of zonation, the way in which plant and animal communities organise themselves in distinct bands, dependent upon their relative abilities to withstand such environmental pressures as salinity, temperature and desiccation etc. The zones here, from left to right,  are brown and green seaweeds on the sandy shore, then on the rocks, green algae, a black lichen, a yellow lichen and then finally the green sward of the salt marsh. Nature just loves a bit of order!