Education and Careers

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.

Walk 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.

This 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.