Geology Walk in the Brecon Beacons

The present is the key to the past and the past is the key to the present.  Fforest Fawr Geopark covers the Western half of the Brecon Beacons and is one of 119 recognised areas of international geological importance in the UNESCO Global Geoparks Network.  Today I joined SWOAPG (South Wales Outdoor Activity Providers Group) along with Alan Bowring, a Geologist for the Brecon Beacons National Park for a geological walk around Llyn Y Fan Fawr and Llyn Y Fan Fach from Tafarn Y Garreg.

Forecasts of gusting winds and heavy rain later in the day didn't dampen our spirits and we set off staying low along the base of Fan Hir, to avoid the worst of the approaching weather.  The river Tawe was our first stop, where we discussed the journey that all rocks are on.  It is easy to forget about the changes still happening when we talk about the Ice Ages that shaped the defining valleys in the Brecon Beacons and the hundreds of millions of years that have created the geology we see today.  But, the Tawe shows change in action and we considered the large rocks in the river, deposited by the traveling glacier and how eventually they may end up in Swansea Bay as pebbles or sand.

 Old Redstone. The different layers show the direction of flow as the sandstone formed and the red colour comes from iron particles in the rock. This image also shows a buildup of quartz on the right.

Old Redstone. The different layers show the direction of flow as the sandstone formed and the red colour comes from iron particles in the rock. This image also shows a buildup of quartz on the right.

The main focus of our day revolved around Old Red Sandstone, the presiding rock type in this area of the park.  We could also see areas of mudstone, limestone, grit and coal measures in the surrounding hills, creating a complex landscape of interwoven geological stories.  We reflected on how the different rocktypes came to exist in this area of the park, with evidence of origins from North Wales and beyond, traveling with the water, snow and ice to leave clues that suggest how the landscape may have once looked.

Alan explained that although it is possible to use all of this evidence to consider the history of the landscape, there are often multiple options and differing ideas that are then associated with it.  By comparing this evidence and the theories stemming from it, with potentially similar areas around the world, we can start to build up more comprehensive pictures of the shaping of South Wales.

Most of what we can see in our present day Beacons landscape was shaped in the last ice age known as the Devensian period, beginning approximately 120,000 years ago.  The Glaciers effectively bulldozed previous evidence of Ice Ages, although Alan suggested that we can assume there have been at least 3 others and perhaps more.  During this time, much of the Brecon Beacons would have been covered in ice around 600m deep in places, leaving the highest peaks such as Pen Y Fan protruding from the sheets.

 Looking along Fan Hir up the valley with the cirque morraine behind us.

Looking along Fan Hir up the valley with the cirque morraine behind us.

Fan Hir is an excellent ridge for visualising the creation of these glaciers, with wind whipping up the snow and depositing it on the steep leeward sides of the North Eastern ridges.  When the ice departed it left Morraines, Tills and Erratic rocks in its wake.  One of these Morraines is particularly characteristic on this walk, directly following the bottom of the ridge.  We considered the different methods for its formation, with one option being that perhaps it was made from rocks falling down across the snow and ice on the edge of Fan Hir or that it was carved out by glacial travel making it a Cirque Morraine. A cirque glacier can also be used to explain the formation of the lakes, creating an armchair in the hills and carving out the basins where the Tarns form.

 Reflecting on Bronze Age Settlements on the edge of Llyn Y Fan Fawr.

Reflecting on Bronze Age Settlements on the edge of Llyn Y Fan Fawr.

As well as glaciers and rivers we also briefly considered human evidence.  With one rather deep circular intrusion possibly suggesting a Bronze Age Settlement, although Alan reflected that this could not really be confirmed without further archeological excavations to provide more evidence.

The day was a real eye-opener for another area that I frequently walk through (see my previous post for learning on the Gower) and encouraged me to look more closely at the hints around us, as everything we look at has a story to its origins.  I found it particularly difficult to comprehend hundreds of millions of years and although it is easy to place these significant shaping forces on a timeline, it is much harder to actually understand these lengths of time.  Making comparisons with a 90 year human life or a calendar year can help to put some of these ideas into perspective.  If Geological history were a calendar year, the most recent Ice Age wouldn't be until 10pm on December 31st and the first direct human ancestors would arrive at about 11pm on the same day. 

Thanks to SWOAPG, Matt WoodfieldAlan Bowring and the rest of the group for a mindblowing, whistle-stop tour through the last hundred million years! 

 Alan's expertise helped to bring the geology of the Beacons to life.

Alan's expertise helped to bring the geology of the Beacons to life.