Why are cliffs striped?

A Post from the Past [2009]

Stripey cliffs? We’re talking rocky shore zonation here – not rock strata.

Some colour-banded Carboniferous limestone cliffs at Worms Head, Rhossili Bay, Gower are shown above.

The layers of rock that form cliffs can lie at any angle – but there are often horizontal bands of colour superimposed on the strata in the inter-tidal region: that part of the shore which is regularly covered and uncovered by the tide.

These stripes vary in colour at different heights above the beach level. They reflect the colonisation of the rock surface by different kinds of organism. They represent groups of plants and animals which are more able or less able to live where they are subject to exposure to air and to sea water.

At the lowest level, closest to the sandy beach, is a mostly black band where mussels, Mytilus edulis Linnaeus, predominate accompanied by a smaller number of limpets Patella spp. attached to any available free surface and dogwhelks Nucella lapillus (Linnaeus) eating the mussels. This zone is only out of the water for a relatively short time at low water.

Next up is a green-grey band where acorn barnacles encrust most of the rock surface and where dog-whelks continue to find plenty to feast on. This zone is exposed to the air for a longer time than the band below.

Above the barnacles is another black coloured stripe. This time it is a smooth coating of black tar-like lichen, probably Verrucaria mucosa Wahlenberg typical of middle and lower shore. There may be also be areas of the  black lichen Verrucaria maura Wahlenberg that prefers the upper shore. In this zone the lichens are only under water for a comparatively short periods.

The highest colour band is a zone again occupied by lichens but these are bright orange ones; basically terrestrial but not minding the odd splash of seawater when tides are high. They are often just below the turf-line. The sort of species found at this level are Calloplaca marina and Xanthoria parietina. As I did not scale the cliffs to get a closer look, I cannot be certain of the specific identifications of the lichen but the ones I have mentioned are characteristic of these shore zones.

Basically, the coloured stripes from low to high tide levels on cliffs and rocky beaches are zones where animals and plants with various tolerances to sea and air exposure can live comfortably.