The Geology of Worms Head by Peter Francis
Peter Francis is a Gower warden for the Countryside Council for Wales

As soon as you reach Rhossili, one of the first things to strike you is the splendour of the sweep of Rhossili Bay. By far the largest of Gower's beaches, its golden sands lie between the two islands of Worms Head and Burry Holms and the cliffs that lead up to them.

It's no accident that it lies here. The rocks that form the two arms of the bay are of a harder Limestone compared with the softer Old Red Sandstone that lies in the middle. The sea has found it easier to erode the softer Sandstone than the Limestone and so had eaten further east into the middle of the bay than into its arms.

Behind the bay rises Rhossili Down to a height of 600 feet. It is the remains of an old beach level much eroded later on by the ice sheets of the last but one ice age. The bench of land below, with the Old Rectory in its middle, is a Solifluction terrace, caused by earth slumping down from above during a cold phase. A Neolithic hand axe - the oldest human artefact found in Wales - was found here and is now on display in The National Museum in Cardiff.

As you walk along the top of the cliffs towards Worms Head, you will notice how level the land is. This is because it is an old beach level raised up to a height of 200 feet since it was originally formed. This is a common feature around the south Gower cliffs.

You will also pass a Promontory Fort as you walk along. This is the remains of an Iron Age fort, built by the local tribes in a time when there was deterioration in climate. This caused a migration of people from the uplands, which were becoming barren and uninhabitable, down towards the more attractive lowlands and especially those of south Gower which, as they were on limestone, were much more fertile than the areas underlain with the coal measures.

All along the Gower cliffs are a series of these forts showing the pressures brought on the local tribes to defend their valuable farmlands. This fort is in the middle sized group of Gower Hill forts. You need to go to north Gower and the forts of Cilifor Top and The Bulwark to see the larger ones.

If you were to follow along the steep footpaths that descend down the cliffs here (they are very steep and traverse over unprotected drops - not for the inexperienced!), you will notice a number of fissure (Vadose Canyon) cave entrances that are filled with a red clay. Although the caves are much older, this red cave fill dates back to the Tertiary era when desert like conditions prevailed. On the cliffs below, just above water level you may find iron rings and the signs of a quay built to load the limestone that was quarried from the quarries that scar the cliffs onto waiting boats. The whole length of the cliffs here were worked in the past as the limestone was a valuable export, taken across the Bristol Channel to Devon and Cornwall to be heated and turned into lime to fertilize the land there, as no limestone is found there. Limekilns can still be seen on the quaysides in places such as Lydmouth as well as along the Gower cliffs, the nearest being in Fall Bay.

If you regain the cliff tops, as you walk along you can look over the restored dry stone wall inland to the medieval strip farming that still continues in the fields there. This is one of only a few Medieval "Vile" Field systems left in Britain.

Eventually you will come to the old coastguard Lookout overlooking the sound (The Shipway) between the mainland and Worms Head. There is a definite geological reason why it has been eroded away by the sea and made Worms Head a tidal island. If you go down onto the rocks you will find a number of small faults in them running out to sea. They are filled with thick bands of white calcite which makes them easy to find. The way you can tell the mineral is calcite and not quartz is by scratching them with a two pence piece. If it were quartz it would wear into the coin as the mineral is harder than it, leaving a pencil type mark on the rock surface. As it is calcite a white powder is produced as the penny is harder than this mineral and can cut into it. Limestone can be dissolved chemically by water and is the only rock that is eroded this way. But as the water does so it quickly becomes saturated with limestone. In some conditions it can become supersaturated so that as it travels along fissures, such as those produced by faults, it leaves some of the mineral behind in the form of the very pure calcite. Iron and other trace minerals form red coloured bands in it. In the fault fissures here this calcite has completely filled the voids.

The reason these faults are here is that they are minor side faults to a much larger fault that runs along the south Gower coast. If you walk further out to sea, going to the left of the causeway, you will see that although the beds of limestone slope inland at 30 to 45 degrees at first, suddenly there is a gap in them, and then they continue, but now sloping out to sea.

You are now in the middle of an upward fold in the rocks caused by the major fault. This is shaped like the hull of an upturned boat and is termed a Pericline (it dips down at both ends - an Anticline doesn't).

It is easier to keep to the right side of the Causeway if you want to cross to the Worm but care must be taken and tide tables consulted as the tide rapidly closes this section. If caught out by it under no circumstances should you try to swim back across as the tide becomes a huge river that will sweep you far out into Rhossili Bay. Deaths have occurred to people attempting to do this.

If you look hard a two metre long ship's anchor can be found encrusted in the rocks here. It is all that remains of a wrecked ship whose cargo of coal kept the local households warm for many a year after it ran aground. On the path down from the lookout you may find the odd fragment of coal that they spilt as they carried this free fuel home.

Look in the many rock pools here and you should find hermit crabs and snake lock anemones, coral weed as well as blennies and other life.

Worms Head itself is shown on the OS map divided into three sections - the Inner, Middle and Outer Heads. In fact they are all joined though the rough tooth like low, rocky section linking the Inner and Middle Heads can be swamped with storm waves at high tide. Seals are resident on the Worm but to see them you must keep quiet and keep a low profile if peering over the cliffs to see them because they'll take fright if they see your profile silhouetted on the skyline. The top of the Inner Head can again be seen to be perfectly flat showing that it was once a beach level the same as that on the shore.

You'll pass a rectangular enclosure as you walk on, thought to be the remains of a medieval farm building though this is uncertain as no excavations have taken place here. Sheep were kept on this part of the island until recently and Swansea University had erected two rectangular enclosures to exclude them, so as to measure the effect they were having on cropping the grass. The grass inside was significantly longer and in thicker mats than that outside. The grasses on the Middle Head, where the sheep couldn't reach was also much denser. Another Iron Age camp can be spotted from the air on the crest of this Head but is invisible from the ground.

Crossing onto the Middle Head you are at once confronted by the Devils Bridge. This is all that remains of a collapsed sea cave and this arch will eventually collapse too, dividing the Middle Head in two. It gives an airy crossing! To the south of this the rocky shore has been completely flattened by wave action, so creating the present wave cut platform. If you go onto it you might be lucky enough to find an octopus hiding in one of the water filled joints. Towards the end of this middle section, to the right of the path, another truncated cave gives a window through which you can look along the north wall of the worm.

Ahead a square cut inlet links this section with the Outer Head. If you are a diver and can visit this area by boat then in the left hand corner of this inlet is an underwater cave that you can enter and do a swim through, re-emerging a few metres from where you started.

You are not allowed to go any further during the nesting season for fear of disturbing the birds, but at other times you can climb onto the Outer Head, passing through a prehistoric midden, mainly of sea shells, as you do so. A short distance further on, up on the sloping bank to your right a narrow vertical band of bare limestone can be seen. If you climb up to it and listen, a rushing sound can be heard and if you put some grass in the narrow fissures there, in a few moments it will be blown forcibly out. Its good fun to get an unsuspecting person wearing a waterproof to sit over it, and, if positioned correctly they can become inflated like a Michelin Man as the hole blows!

For years it was thought that this was caused by a large swell entering and filling the large cave that is on the other side of the Head. This cave, in really rough weather emits a spout of spray high enough to rise completely over the crest of the Head and in calmer conditions is large enough to take small boats such as canoes into. It is possible to canoe in, become sealed inside when the next swell passes before re-emerging in the trough. During the time you get sealed in an eerie green light shines up through the water! This is also an exciting place to dive into and during certain times of the year the whole of the rock surfaces underwater can be covered with hundreds of large spider crabs making everywhere a moving tangle of legs and bodies!

During last year I found another smaller cave that is wholly underwater about halfway back towards the low neck that connects these two Heads. On diving into it a tall thin passage was followed (a Vadose like rift) that went in some distance, gaining in height all the time. Although it was a calm day, as I went further in the underwater swell increased dramatically so that I was soon being thumped into the walls. In the end it was getting very unpleasant and only with some difficulty was I able to manoeuvre backwards out of it. (In anything but the calmest conditions this could become extremely dangerous as the swell is capable of hurling you forward several metres and if the passage were to suddenly close down you could become permanently wedged if using a back mounted cylinder. The passage was not pushed to its end but must be the one that links with the blow hole on the surface.

The walker can go on up the rough scramble to reach the very end of the Worm and enjoy the panoramic views at its end. For most the walk ends here but if you are a climber, used to heights and well equipped, it is possible to scramble down the cliff at the very end of here. (If you have no climbing experience then you should be very careful as the route finding down it is not obvious and if you get it wrong then you could end up over some very nasty drops a long way from help - remember that the tide will be coming back in soon so it's not a place to get stranded on!)

If you are successful in climbing down it you will reach a broad ledge that descends south to the sea. There are two cave entrances here and prehistoric human remains have been found in them - an incredible burial site! Providing there isn't too big a swell you can escape at the foot of this ledge, climbing a low cliff to get back to the path.

If you've still enough energy left on returning to the mainland you can continue on around Tears Point into Fall Bay. At the head of the gently sloping beds of rock that make up the fore shore here you will find a layer of rounded limestone pebbles naturally cemented together. If you search carefully in this layer you will find many limpet shells. This is the remains of a fossil beach level, now ten metres above the present beach. It is called a "Patella Beach" after the Latin name for these shells. (If you look carefully at them they look similar to a knee bone - the patella bone).

Immediately above this beach layer is a thick layer of jagged, angular rocks. This is the remains of an Ice Age deposit and as it sits on top of the beach layer, must be more recent than it. If it is from the last ice age then the beach layer must be from the warm period before that. At the interface of these two layers the teeth of Straight Tusked Elephants have been found in this bay. Gower was home to many cold and warm period animals and some of their remains are on display in Swansea Museum.

Each bed of limestone here is only about a metre thick. They are covered with fossils of corals, crinoids and mud casts of giant worms.

If you look across the bay the headland of Lewes Castle stands out prominently. If you count the horizontal cracks (bedding planes) that mark out each bed of limestone there, you will find that it consists of only four layers despite being two hundred feet high.

The thickness of these beds made it much harder for the sea to erode, leaving it standing out much higher than the thinly bedded rocks.

You can follow the path at the top of the cliffs, back to the lookout and on to Rhossili or go on to the head of the bay before returning through the fields to Rhossili.