The Age of the Landscape

Top map:  the area of North Pembrokeshire which has been most affected by glacial processes (thanks to Prof David Bowen)

Middle photo:  Part of the area of thick fluvuioglacial sands and gravels, between Newport and Cardigan (the two gravel pits are Trefigin and Pantmaenog)

Lower photo:  Part of the Gwaun-Jordanston meltwater channel system, at the western end of the Gwaun Valley.  The big channel at the bottom of the photo is the Nantybugail Channel -- it can also be picked out on the map.

There have been several comments recently on this blog about the age of the landscape and the age of land surfaces, with some contributors apparently rather keen on the idea that a few thousand years ago there are valleys where there were once hills, and that some low-lying areas have suddenly been uplifted.  Well, let's get one thing clear -- the landscape of Pembrokeshire (and this goes for most of the British landscape) is for the most part very old indeed.  I'm talking about a landscape which has not changed much over millions of years, apart from superficial vegetation change and the introduction of new features created by massive civil engineering projects (motorways, water supply reservoirs, ports and harbours, airports etc).

The only places where we have had dramatic changes are around the coast, where cliff falls, landslides etc triggered off by marine erosion (especially on high-energy coasts) can occur, and where high rates of sedimentation can change the outlines of embayments and estuaries.  In Pembrokeshire, for example, we can see how some Iron Age promontory forts have been largely whittled away by coastal erosion over the course of 2,000 years or less.  But right next door there may be a bit of coast that has incredible stability and longevity, where there is less exposure to wave attack.

We know from coastal  and valley sediment sequences that most of inland Pembrokeshire hasn't changed much for more than 2 million years.  In other words, the basic pattern of hills, valleys and plateau areas is the same now as it was then.  But in the north of the county (see map at top) glaciation has wrought major changes in both surface altitudes and in the arrangement of rivers and valleys.  The big meltwater channels to the south of Fishguard -- and including the Gwaun Valley -- appear to be NEW features created initially underneath the ice -- maybe during the Anglian Glaciation around 450,000 years ago.  The valleys (which are in places more than 100m deep)  have been modified during subsequent glaciations.  There have also been major changes in the drainage pattern in the Teifi and Nevern Valleys, and in the interfluves where meltwater channels have been created subglacially and by overflowing lakes.  Some old valleys have been abandoned, and filled with glacial deposits, and other new valleys have replaced them.  Glacial, fluvio-glacial and lake deposits are is some places hundreds of metres thick.  There are hummocky moraine deposits, kame terraces and eskers -- two of which are currently exploited for sands and gravels for the building industry.

How old are these landscapes of sedimentation?  For the most part, Late Devensian, which means c 20,000 years old.  So these are Pembrokeshire's "young" landscapes.  Further to the south, in South Pembrokeshire, we seem to be looking at landscapes which may be well over ten million years old.....