10 Reasons Why …. 10 Rheswm Pam …. : 1

Monday 30th March 2015

Blog post 1 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/

10 reasons Welsh

Inspired by a trip into the mountains of Snowdonia in snowy January, Hywel Griffiths (http://hywelgriffiths.blogspot.co.uk/) and I decided to initiate a Welsh translation of the ‘10 Reasons Why Geomorphology is Important’ brochure (see earlier blog post). A request for sponsorship from the British Society for Geomorphology (BSG) for the translation, design and printing is pending. If successful, this will enable us to target a wider cross-section of the public with the message. To highlight this initiative, in the remaining 10 months of 2015 Hywel and I will be providing examples of Welsh landforms and landscapes that illustrate the 10 reasons, in both Welsh and English. These examples will help to demonstrate how many geomorphological issues are represented right here in Wales. In particular, although some aspects of present and future environmental change pose threats to Welsh landscapes, past environmental changes – including some extreme events – have been responsible for the shaping of some of the country’s most treasured scenery.

Reason 1. Landscapes are shaped by movements of mass. Landforms are shaped by geomorphological processes, which essentially involve the movement of mass ‒ rock, sediment, water ‒ across the Earth’s surface.

The mid Wales uplands have been subject to a long, complex and imperfectly understood history of tectonic uplift, rock weathering, periodic erosion by ice sheets and glaciers, and various slope and valley-floor geomorphological processes. This view of the upper Elan valley (flow from upper left to lower right) in the Cambrian Mountains near the Ceredigion/Powys border, shows how the associated movements of mass have resulted in generally subdued upland topography. Many mountain tops and hillslopes are characterised by rounded, smoothed forms, while valley floors have partially infilled with a mixture of weathered and transported rock (gravel, sand, silt, clay) and peat. Rivers like the Elan are gradually reworking some of the valley-floor deposits, transporting them farther downstream. As can be seen below, within this subdued upland topography, local examples of more spectacular movements of mass can be found (Photo: Stephen Tooth).

The mid Wales uplands have been subject to a long, complex and imperfectly understood history of tectonic uplift, rock weathering, periodic erosion by ice sheets and glaciers, and various slope and valley-floor geomorphological processes. This view of the upper Elan valley (flow from upper left to lower right) in the Cambrian Mountains near the Ceredigion/Powys border, shows how the associated movements of mass have resulted in generally subdued upland topography. Many mountain tops and hillslopes are characterised by rounded, smoothed forms, while valley floors have partially infilled with a mixture of weathered and transported rock (gravel, sand, silt, clay) and peat. Rivers like the Elan are gradually reworking some of the valley-floor deposits, transporting them farther downstream. As can be seen below, within this subdued upland topography, local examples of more spectacular movements of mass can be found (Photo: Stephen Tooth).

Nant Cwm Du, Cambrian Mountains, Ceredigion. This northward draining, left-bank tributary of the Afon Ystwyth (flowing left to right at the bottom of the photograph) is incised into large postglacial landslide deposits. Numerous movements of mass are visible here. Sometime after the valley glacier receded after the Last Glacial Maximum, leaving unsupported valley sides, a landslide moved sediments across the valley floor, forming the large landform in the middle of the photograph (on which trees have grown and some ruined buildings can be seen) and creating the ‘cwm’ at the top of the photograph. Over time, the Nant Cwm Du has eroded this landform, creating a deep and narrow valley and moving boulders, cobbles, pebbles and finer sediment into the main Afon Ystwyth. Within the Nant Cwm Du valley, large floods have transported cobbles and boulders and deposited them as boulder berms on the small floodplain (an example is visible on the left bank of the stream, immediately upstream of the small track). These smaller-scale landforms can provide valuable information on the timing and size of historical flood events (see Foulds, S.A., Griffiths, H.M., Macklin, M.G. and Brewer, P.A. 2014. Geomorphological records of extreme floods and their relationship to decadal-scale climate change. Geomorphology, 216, pp. 193-207.). Other movements of mass are indicated by scree slopes on the steep back valley wall and by small terracettes (or steps) on the valley sides above the banks of the Afon Ystwyth that indicate soil creep (Photo: Hywel Griffiths).

Nant Cwm Du, Cambrian Mountains, Ceredigion. This northward draining, left-bank tributary of the Afon Ystwyth (flowing left to right at the bottom of the photograph) is incised into large postglacial landslide deposits. Numerous movements of mass are visible here. Sometime after the valley glacier receded after the Last Glacial Maximum, leaving unsupported valley sides, a landslide moved sediments across the valley floor, forming the large landform in the middle of the photograph (on which trees have grown and some ruined buildings can be seen) and creating the ‘cwm’ at the top of the photograph. Over time, the Nant Cwm Du has eroded this landform, creating a deep and narrow valley and moving boulders, cobbles, pebbles and finer sediment into the main Afon Ystwyth. Within the Nant Cwm Du valley, large floods have transported cobbles and boulders and deposited them as boulder berms on the small floodplain (an example is visible on the left bank of the stream, immediately upstream of the small track). These smaller-scale landforms can provide valuable information on the timing and size of historical flood events (see Foulds, S.A., Griffiths, H.M., Macklin, M.G. and Brewer, P.A. 2014. Geomorphological records of extreme floods and their relationship to decadal-scale climate change. Geomorphology, v.216, pp. 193-207). Other movements of mass are indicated by scree slopes on the steep back valley wall and by small terracettes (or steps) on the valley sides above the banks of the Afon Ystwyth that indicate soil creep (Photo: Hywel Griffiths).

Did You Know?  In the UK context, Wales is characterised by high ‘relative relief’, defined as the difference between the highest and lowest elevation for a given area. For instance, the highest point in Wales is the summit of Snowdon in Snowdonia National Park (1085 m), but this mountain is surrounded by deep, steep valleys that descend sharply to narrow coastal lowlands. Other parts of Wales possess similarly high relative relief. For nearly two centuries, though, most attention has focused on the identification of the high-level erosional surfaces (‘denudation surfaces’) that characterise much of Wales. New approaches using satellite data and computational mapping methods show that the country as a whole is characterised by four widespread denudation surfaces at 560–500, 430–370, 245–155 and 100–45 m above sea level. Identification of these surfaces is an important first step towards improved understanding of the geomorphological history of Wales (Source: Rowberry, M.D. 2012. A comparison of three terrain parameters that may be used to identify denudation surfaces within a GIS: A case study from Wales, United Kingdom. Computers and Geosciences, v.43, pp.147-158).

A light dusting or something more thorough?

Thursday 5th March 2015

I’ve never really been a fan of The Daily Telegraph or The Sunday Telegraph newspapers. The right-leaning editorial biases and conservative views on many issues grate with me, and one particular contributor’s opinion pieces and blogs on climate change topics (let’s just call him J.D.) make me want to reach for a large bottle of whiskey with the same initials.

But credit where credit’s due. An online article in The Telegraph from 4 March 2015 has a fascinating short video clip that illustrates the transport of dust from the Sahara to the Amazon on the other side of the Atlantic Ocean: http://www.telegraph.co.uk/news/science/11450232/Incredible-video-explains-how-the-Sahara-fertilises-the-Amazon-rainforest.html#disqus_thread

A static screen shot captures the essence of the video, which is based on data collected by NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite from 2007 through 2013.

The path of the dust clouds blown from the Sahara to the Amazon basin (highlighted in green) and other parts of South and Central America

The path of the dust clouds blown from the Sahara to the Amazon basin (highlighted in green) and other parts of South and Central America

The full set of findings have been published online by Yu and colleagues in Geophysical Research Letters (accepted article, DOI: 10.1002/2015GL063040) but the video and accompanying short article bring some of the key facts and figures more prominently into the public domain. About 182 million tonnes of dust is blown off the western edge of the Sahara each year. Some of this dust falls from, or is rained out of, the atmosphere into the Atlantic. But on average 132 million tonnes makes it to the eastern coast of South America, whereupon the Amazon receives its average yearly share of about 27.7 million tonnes. The Amazon dust contains about 22 thousands tonnes of phosphorous, an essential nutrient that acts as a fertilizer for the rainforests. Most of the phosphorous comes from the Bodélé Depression in Chad, where an ancient, now desiccated, lake bed is exposed to vigorous wind erosion. This amount of phosphorous is roughly the same amount that is washed away from the Amazon basin by rain and floods every year, suggesting that the African dust plays a key role in preventing phosphorus depletion from the Amazon over decades to centuries.

As The Telegraph article states, the findings ‘show how one of the planet’s driest places is helping sustain one of its most fertile’. There’s obviously much more to it than that, for as the last line of the article hints, the findings are ‘part of a bigger research effort to understand the role of dust in the environment and on local and global climate’. In more expansive, technical terms, dust is a key, but still poorly understood, component of global biogeochemical cycles and the climate system, including possible influences on hurricane generation and suppression. There’s also the debate about whether a future ‘greening of the Sahara’ would necessarily be a good thing. If the current global warming trend continues, then with warmer seas and more intense heating of the Sahara, there’s a chance that more and more rain will penetrate further and further into the heart of the Sahara, perhaps enabling the return of lush wetlands, rivers and lakes. Such features have characterized the Sahara many times in the past, most recently up until about 5-6 thousand years ago.

So why is this a bad thing? First, when the Sahara is green and wet, dust transport off North Africa and across the Atlantic is much more limited, so phosphorous shortages may limit the productivity of the Amazon. Second, there have been some suggestions that when the Sahara is better watered, the Amazon may become much drier. In this complex, teleconnected world, a Saharan expansion of wetlands, rivers and lakes might coincide with widespread desiccation and burning off the Amazon rainforest. This would have widespread implications not only for biodiversity but also for the oxidation and loss of substantial amount of organic carbon to the atmosphere. Along with the rampant human contributions of greenhouse gases to the atmosphere, this is something that the world can ill afford.