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Caswell Bay

Follow the Newton Road (B4593) from the mini-roundabout at Oystermouth and follow the signs for Caswell. It is a tortuous route, but after about two miles you will descend into a deep valley. The road here skirts the beach and the CAR PARK is entered on the right. It is large, but Caswell is one of the most popular beaches of Gower and it needs it. In summer, therefore, be sure to get there early or else go in the evening.

So far, in discussing the varied geology of Gower, we have been dealing with large rock-units like the Carboniferous Limestone and the Coal Measures, since on this scale it is possible to pick out the major features of landscape to which these major units give rise. However they may be subdivided, for greater accuracy of observation, into sub-units, and this is especially true of the Carboniferous Limestone.

I have already mentioned the lowest of its subdivisions, the Lower Limestone Shales: and the rocks which you inspected at Clement's Quarry in Oystermouth lie at the opposite end of the succession, and are consequently known as the Upper Limestone Shales.

In between these two end-members, there lies a wide range of very different limestone types, some thick, some thin. The differences which divide them are more subtle than those by which we differentiate, say, a shale and a conglomerate; but once you have seen them and 'got your eye in', as geologists put it, you will never be able to think of limestones again as merely 'light grey and all the same'.

What are limestones? Rocks like sandstones, shales and conglomerates are made up of particles of pre-existing rocks broken off by erosion and redeposited as new sediment. They are the rubbish of erosion, and the rubbish consists, by and large, of the most resistant minerals, best able to withstand the weather. They are called 'detrital' rocks, from the Latin detritus, meaning 'that which is worn off'. They occur in nearly all areas of the world, in seas, rivers, on the land, under glaciers, - everywhere!

Limestones, on the other hand, form only where this detritus is scarce, which usually means either far from land or bordering land which is low-lying. They also form most prolifically in the tropics; modern limestones are being made in the Bahamas, the Persian Gulf, and in the Pacific.

This simple fact, that limestones form a long way from Europe, has been at least partly responsible for the relatively late stage at which geologists have come to understand them. It was a difficulty with uniformitarianism itself, since being unfamiliar with the processes that govern limestone formation in the modern world, our speculations about ancient limestones were necessarily mistaken and restricted. Now, happily, many geologists specialise in these fascinating, subtle rocks.

Limestones are made up of one dominant mineral, calcite - a form of calcium carbonate, or lime. But limestones are far from simple to understand, because this variable and plastic mineral can assume a huge range of other forms, governed by slight changes in the environment of deposition or in the conditions of burial.

The immense variety of living things, which are always very much a part of limestones, adds another dimension to their complexity. Moreover, limestones can cement and become hard rocks rather than loose sediments in a few tens or hundreds of years. This can happen (and usually does happen) at or near the surface. So the nature of the cement which joins the particles together is also telling us about conditions either at deposition, or else very soon thereafter. But for this kind of study, a microscope is required.

CaswellWalk over what was a storm-beach (now covered in concrete below the shops) and make your way to the centre of the bay. Here you will discover that Caswell is divided in two by a short, low promontory (picture). From here, you will get a good impression of the whole beach.

Notice that the rocks of the promontory itself are dipping away to the north. But if you look over to the western cliff (Redley, in the middle distance in this picture), you will see that there they are dipping south. This tells us that there must be an anticline between the promontory and the cliff. In fact, it is the Langland Anticline, whose termination we saw in the rocks of Bracelet Bay (Tour 1, stop 5).

So why are there two bays at Caswell? The answer is given by the existence of two neighbouring faults, running approximately north-south, one along Caswell Valley (beneath the car park) and another directly under that block of expensive apartments behind the western arm of the Bay.

At the foot of the western cliff (called Redley Cliff) runs a small brook, which originates at a spring nearby. In times gone by, water from this stream was stored in the massive concrete cistern beyond and supplied to the houses of the bay by gravity-feed from a wind-pump situated on the top of the cliff. Should you scale this height, you will discover the pump's footings at the summit.

Now head over to the first exposure on the eastern side of the bay, just below the shops. Here there is a light grey, hummocky surface dipping north, and above it lie some less resistant black and dark grey limestones, in rather thinner beds.

Whiteshell PointThis is the thinly-bedded unit, and the hummocky bedding-plane beneath it is the top of the limestone which we shall call the Caninia Oolite. Caninia is a fossil coral found in this unit, and the sediment itself consists of minute beads of lime about the size of a pin-head. These are ooliths, and any rock composed of them is called an oolite.

In the photo above, the bright limestone to the left of the picture is the Caninia Oolite, coming to surface from beneath the syncline described below. The headland (Whiteshell Point) shows the characteristic seaward dip of South West Gower headlands, whose constituent rocks dip south (see picture below also).

We shall say more about these two units in Three Cliffs Bay (Tour 4); but for the moment, divert your attention to the rocks directly opposite you, on the far bank of the stream where there are some strange buildings. Can you see the Caswell Bay Mudstone? It is not where it should be, which tells us that there has been some displacement along the fault running through the bay. See if you can discern the Mudstone (it is partially hidden by the buildings) and try to decide what sort of movement could account for such a displacement.

Get to know the appearance of the Caninia Oolite, because it is a valuable marker-horizon, as is the mudstone below it. Firstly, note its pitted and hummocky top surface; in Three Cliffs Bay we shall see this again, and an explanation will be offered. Note also the close jointing which breaks the bed into small blocks, giving it a massive appearance. And if you use a lens, you will be able to see the ooliths.

These are beads of lime formed around a central nucleus (such as a fragment of shell, or a sand-grain). The layers of lime are concentric, like the leaves of an onion - though all you will be able to see with your lens is their smooth, polished exterior. If they appear rough, this is due to weathering. Ooliths are characteristic of shallow carbonate sandbanks close to the zone which is affected by wave-action at surface. This continual agitation moves the ooliths about and wears the precipitates that form on its exterior into a perfect, bead-like sphericity.

Walk now along side the cliff, not keeping too close too the rocks. This will ensure that you get a broad picture of their colour and jointing. Keep your eye on the direction of dip of the bedding planes, and be sure that you are not confused by the tide-mark, or the discolouration below it. Always judge a rock's weathered colour by its appearance above this zone. When you think the Caninia Oolite has passed downwards into another rock-type, stop.

You should now be a few yards further on. The rock has changed, and become less strongly jointed, more recognizably bedded and a lot darker. Close examination will reveal that it is fairly featureless and free of ooliths, with a sugary sparkle in sunlight.

We shall call this lithology the Laminosa Dolomite: Laminosa being another characteristic fossil, and dolomite being the principal mineral composing the lithology. Dolomite is calcium magnesium carbonate (CaMg(C03)2). Most calcite (CaCO3) contains a little magnesium in the natural state, but here in dolomite, the magnesium is present 'officially', as a regular and necessary part of the crystal lattice. Dolomite is usually formed as an alteration-product of ordinary limestone, and this example probably owes its origin to a time when, some time after initial burial, there was a withdrawal of the sea. This would have exposed the more coastal limestone sediments to the air, and hence also to the influence of fresh water (rain).

Under the land, water lies at depth, draining down from the surface until it reaches a level where the pores in the rock are completely full. This level is called the water table. In coastal areas, this body of fresh water must mingle seawards with the salt water that fills the pores of the sediments under the sea floor. Recent research has shown that, in those areas where limestone is being made, the chemical composition of this 'mixed' zone encourages the limestones to change into dolomite by the absorption of magnesium.

So, when there is a fall in sea level, fresh water accumulates in recently-formed carbonate sediments close to the shore. In the diagram, a limestone island has been formed, and a lens of fresh water is forcing back the salt along a wide front of mixed, brackish porewater. For this reason, we often find dolomite at some depth beneath evidence of regression. The evidence in this case consists of the Caswell Bay Mudstone, which formed at or about the tide-mark of the Carboniferous sea. The hummocky surface upon which it rests is also conclusive evidence of sub-aerial exposure due to regression (see Tour 4).

Walk on down the beach until you recognise the Caninia Oolite reappearing. Then, standing a good thirty yards away from the cliff, read on.
The dolomites have been dipping constantly northwards all along, yet here is the Caninia Oolite once again, when one should expect to see the next unit below dolomite in the sequence.

The reason for this is the presence of the Caswell Thrust. Using the diagram, you should be able to see where the fault-plane emerges from the cliff, in a gully immediately north of the brilliant white oolite. This thrust is running east-west like all the others, but it is exceptional in that the northern block has ridden up over the southern; as a result, this fault-plane dips northwards instead of south.

If you follow the bedding of the dolomites into the zone of the thrust, you may see that several appear bent over, and splayed at their ends. This comes about when the shearing forces first deform the rocks along a kink-band, as a prelude to full mechanical failure.

Still maintaining a fair distance from the cliff, walk on to the 'corner', where the bay opens out suddenly to the west. Then pause and look back towards the 'angle'. There, picked out by the narrow bedding of the mudstones, you will see a tight little syncline. If you cannot see it, you are probably too close.

On the southern limb of this little syncline, the beds bend steeply upwards, to become vertical as they rise toward the crest of the next upfold. Depending on the state of the vegetation, you should be able to see these vertical strata in the slopes of the headland, picked out by the outcrops as they poke through the thin vegetation.

The neighbouring upfold is a very "tight" anticline, so-called because the angle between its limbs is small. Dolomites lie in its core, and you should be able to make out this anticlinal hinge by watching the dip of the dolomites as you walk towards the eastern headland (Whiteshell Point). There is a deep gully, floored with pebbles, across which the facing strata change from being northerly to southerly dipping. But you will need to be careful, because the beds on both sides are very close to vertical, and this could make it difficult - especially in view of the uniformity of lithology. This upfold is the Langland Anticline.

Once you are satisfied that you have identified the core of the fold, walk along the line of the rocks as far as the tide will allow, and note the gradual decrease in dip. You are now walking away from an anticlinal hinge, so the rocks are getting younger. You should therefore expect to see the reappearance of Caninia Oolite, Caswell Bay Mudstone, and other formations younger still, which we have not yet seen. If the tide does not allow you to do this on the beach, there is a cliff-path above the rocks which will take you right to Whiteshell Point, and beyond.

Indeed, the Caninia Oolite does reappear: but yet another thrust (dipping conventionally south) causes the repetition of the Laminosa Dolomite above it. After this minor hiccup, however, the succession is allowed to resume its natural course. This includes rocks younger than the mudstone, which lie beneath the shops, road and car park in Caswell itself. Nevertheless, you have seen them before; they are the rocks of Bracelet Bay and Mumbles Head, and here they form the tip of Whiteshell Point.


West GowerIn Caswell, then, we see the distilled essence of Gower geology; a bay eroded along two north-south faults, with two examples each of east-west trending folds and thrusts. You now understand the framework of the entire geological structure of Gower.

You can return to the car park either along the beach, or the path along the headland. Alternatively, there is a very pleasant walk around the end of Whiteshell to Langland, where there are many amenities. On your way back from Langland, you might take a path which leads across the top of the headland (leaving the coastal path just south of the kissing gate) which affords fine views.

If the day is fine, the view to the west (picture, above) shows the distinctive similarity of form among the main promontories of south east Gower, Oxwich Point (in the distance) and Pwll-Du Head sharing with Whiteshell a characteristic seaward taper. This is quite coincidental, but the reason for it is geological in that the rocks that compose them all happen to dip towards the sea. West Gower

By contrast, the cliffs of west Gower (picture, right), from Port Eynon to Rhosili, all dip landwards into the Port Eynon Syncline. Their form is therefore rugged, castle-like and sheer. So it is that each of these two stretches of coastline holds within it a striking and deeply pleasing unity.

Read next tour...Three Cliffs Bay