Pembrokeshires Landscape Today

The land on which Pembrokeshire is situated today was created approximately 60-50 million years ago.

It was created by movements of the plates making up the Earth’s crust, caused by a massive up-welling of molten rock (magma). This led to an early phase of doming and associated rifting which ultimately produced the north Atlantic Ocean. Some of the uplift may have been caused by block movements resulting from the distant collision of continents which created the Alps. Whatever the causes, the result was that a former seabed emerged as a series of stepped and gently inclined plateau surfaces with isolated hill masses that had originally been islands.

During and after the uplift, natural processes were responsible for shaping the landscape we see today. The sea immediately began to create a new coastline and an early phase of surface processes was dominated by intense and deep weathering under warm and humid conditions.

Streams and rivers rapidly cut through the soft sediments which had originally accumulated beneath the sea and a drainage system was superimposed on the underlying rocks. Masses of rock which resisted the intense weathering were later exposed, producing the prominent tors that are now a common feature of the Pembrokeshire landscape such as Carn Llidi and Penbiri to the north of St Davids, Garn Fawr on the Strumble Peninsula, and numerous smaller examples in the Preseli Hills.

On several occasions during the Ice Age, glaciers which formed in mountainous areas surrounding the Irish Sea combined with sea ice to create an ice sheet which moved towards and covered parts of Pembrokeshire. There is evidence that on at least one occasion this Irish Sea Ice Sheet passed south-eastwards over Pembrokeshire and reached the southern coastline of the Bristol Channel area.

It is possible that earlier glaciations were at least if not more extensive, but the signs of this have been removed by other processes. Many geologists believe that movement of ice carried the dolerite ‘bluestones’ used at Stonehenge at least as far as Somerset, from where it would have been relatively easy for Neolithic people to transport them to Salisbury Plain.

Although the climate in general was significantly colder than at present, at times there were marked improvements. Each time this happened, vast volumes of glacial meltwater were produced, flowing on or under the decaying ice, and depositing sands and gravels at its margins. The meltwater also cut steep-sided channels in the land surface beneath the ice, sometimes exploiting pre-existing river valleys. It was also capable of breaching watersheds or cutting completely new channels. The system of channels around the Gwaun Valley is a spectacular example of this type of feature, but most valleys in Pembrokeshire show some evidence of modification by glacial meltwater flowing off surrounding plateau surfaces.

In contrast to the generally held impression of intensely cold conditions throughout the ‘Ice Age’, the climate around 125,000 years ago was, in fact, a little warmer than at present. As a result, there was less polar and continental ice and the sea level was several metres higher. Remnants of wave-cut rock platforms formed at this time can be seen around the Pembrokeshire coast, sometimes with surviving beach sediments. There are notable examples at Poppit Sands (to the west of the main beach), above the cave named Ogof Golchfa (just to the west of Porth Clais), and on the eastern side of Broad Haven South.

The most recent glaciation, which reached its maximum about 20,000 years ago has left ample evidence. For example, the cliffs backing Abermawr and Druidston Haven beaches contain sediments which were scraped off the sea-bed by the advancing Irish Sea Ice Sheet and left behind when it melted. These characteristic blue-grey clays contain fragments of shells of sea creatures that live in waters much colder than the Irish Sea is today.

At the end of the Ice Age, the sea level rose as the polar and continental ice cover melted, reaching its present level about 3,000 years ago, and drowning the lower parts of valleys which had been enlarged and deepened by meltwater. The Milford Haven-Daugleddau estuary is a particularly impressive example of this type of feature, known as a ria, and there is another good example at Solva.

Rising sea levels also reworked large quantities of glacial debris and created the many storm beaches around the Pembrokeshire coast, the best example of which is the pebble bank at Newgale. The first phase of the development of some of the most extensive dune systems around the coast, eg. at Freshwater West, also occurred at this time.

Although marine erosion has been a major influence on the shape of the coastline, it should be remembered that that many coastal features (eg. the ‘Green Bridge of Wales’) have resulted from several different phases of landscape development.

The wide variety of rock types, together with landforms created by the varied processes described above, have provided the diversity of landscape for which Pembrokeshire is justly famed.