Rhossili Cliffs
The geological history of Rhossili Cliffs stretch back roughly 350 million years. The cliffs were formed during the early Carboniferous Period, when the land that would eventually become Wales lay much closer to the equator.
Instead of the windswept Gower coast we know today, this area was covered by warm, shallow tropical seas. Calcium carbonate accumulated on the seabed from both biological material and chemical precipitation, eventually becoming thick sequences of limestone. Across South Wales these deposits belong largely to what geologists now call the Pembroke Limestone Group.
Much of the limestone is full of evidence for that vanished marine world. Crinoids - marine animals sometimes nicknamed “sea lilies” - were particularly abundant, while brachiopods, corals and other shelled organisms also contributed skeletal material to the sediments. Some beds forming the cliffs are dominated by fragments of these organisms.
Other layers tell a slightly different story. The Gully Oolite, for example, consists largely of tiny rounded carbonate grains called ooids. These form when calcium carbonate builds up in concentric layers around a small particle while it is repeatedly rolled around in warm, agitated, shallow water. Their presence provides geological evidence of energetic shoals in the ancient Carboniferous sea. The limestone forming the cliffs were originally deposited in approximately horizontal layers. Hundreds of millions of years ago, enormous tectonic forces affected southern Britain. Gower's sedimentary rocks were compressed, folded and faulted.
Rhossili is an especially good place to see the consequences of these immense forces. The area contains major east-west-trending folds and associated thrust faults. The rocks of Worm’s Head lie on the southern limb of one of these folded structures and consequently dip steeply southwards.
This folding also explains an apparent geological puzzle in the landscape. Rhossili Down, rising immediately behind the village, is not simply a taller version of the limestone cliffs. Its core is made from older Devonian Old Red Sandstone, brought towards the surface within the Rhossili anticline. The limestone forms the flanks of that folded structure and extends out into the headlands, including Worm’s Head.
So when you look from Rhossili Down towards Worm’s Head, you are effectively looking across part of the eroded remains of a huge geological fold.
Once uplift and erosion exposed the limestone, a new phase of landscape-making began.
Limestone is both hard enough to produce spectacular cliffs and chemically soluble enough to develop cracks, fissures and caves. Rainwater absorbs carbon dioxide and becomes weakly acidic. Over long periods it can dissolve calcium carbonate and enlarge natural joints in the rock.
The sea exploits those same weaknesses.
So, while the cliffs may feel permanent on the scale of a human lifetime, geologically, they are transient.
Rhossili Cliffs are more than a spectacular viewpoint. They are a place where a tropical Carboniferous seabed has been folded by continental collision, exposed by erosion, and cut into cliffs by the modern sea.
What looks like one landscape is really the accumulated result of hundreds of millions of years of change. With a history like that, it is little wonder that their scale and drama strikes awe in the hearts of all who visit them.