Limestone Pavement, Carnau Gwynion
Another image from the Beacons this week, and one linked to geography fieldwork (yet again!). I’ve blogged about limestone pavement before, with my previous example being something of a classic, representing subaerial weathering of a fairly tough limestone (i.e. a fairly gradual process with the rock exposed to wind, sun and rain). Here in the Beacons we’re looking at what’s called fretted pavement, which means that most of the solution and breakdown of the bedrock probably went on under a layer of soil. Water, enriched with carbonic acid produced by the roots of plants, attacked the limestone more thoroughly, insidiously working its way into any pockets of weakness within the former seabed. When that layer of soil was stripped away by the passage of the welsh ice sheet, the limestone was then exposed to the damp, cold conditions that followed (it’s 350m above sea level here) and other forms of weathering could play their part, leaving a crumbling, irregular pavement, nothing like the geometric patterns of the Burren. Then humankind came along, and the site became a place of settlement -the green area in the middle is actually the remnant of a ring fort. Mankind stripped further fragments of rock away, clearing the land for building on, and later for turning in to lime for agriculture and construction (there are lime kilns at the base of a nearby escarpment). It’s another great layered landscape; today the prime uses of the landscape here are agriculture and tourism (this is something of a hidden gem -Carnau Gwynion is very close to the well known waterfalls country, which draws thousands of visitors each year).
The reason it’s good for fieldwork is the wide range of landforms in a relatively small area -you can study the pavement itself, but there are also shakeholes nearby and in places it’s clear there has been a massive collapse underground to form a doline or polje. Alongside that, there are sandstone erratics on pedestals that can be used to measure erosion since the end of the ice age, and areas where heather (a calcifuge, or lime-hating plant) grows, indicating extremely rapid drainage and little mineral content in the soil. So, plenty to discuss, and combines nicely with a walk in the Mellte valley to look at a few of the waterfalls to discuss further the influence of geology on landscape, plus sustainable tourist management in a honeypot site… All good geography! Sad thing is, this doesn’t apply to so many of the current exam specifications any more, so it’s less common than it used to be.
If you want to buy this one, be sure to use standard print dimensions. I’ve got a copy on my wall printed on A3 paper, which I think looks great!
All images © Stephen Tyrrell, Mountains, Wild Places & Water, 2013. All rights reserved.