Geology - The Soils

Note added 29/04/2009: Linked Figures and Maps are not yet available for these pages.

Take the road from Senni over to Crai and walk up the track, closed by a heavy gate, which heads south over the Cnewr estate towards Fan Gyhirych ●14 (with thanks to the generous access policy of our neighbours at Cnewr). Look closely at the ditches on both sides of the track as you walk up the hill.

For the first 500 yards the soils exposed in the ditch sections are a dull, slightly bluish-grey and the vegetation contains quite a lot of plants that like plenty of water, such as the common rush Juncus (fig 1.1. ). When the first track leads off to the left and into the forestry, however, the soils change abruptly ●15 (fig 1.2. ) and the vegetation changes to acid-tolerant grasses such as the moorland mat grass nardus stricta. Soon after the forest gate you can see an excellent section, exposed by the ditch to the south of the track, where the soil is divided into three bands – dark at the top, light in the middle and brown below. Continue along the track and, after a small quarry, and a couple of bends, a rather larger bank of soil and rock has been exposed ●16 where it has been cut into by the track. Here you can see the pattern of black, light and brown overlain by repeated bands of light and dark (fig 1.3. ).

Now head back towards Senni and look at the soils exposed in the road ditch just east of Pen-waen-dwr ●17. This soil is largely uniform red-brown right from the till at the bottom to the root-mat at the top (fig 1.4. ).

Finally, have a look at any soil you see exposed in the valley itself – for example in the sections cut by the river by the Heol Senni bridge ●18 (you can climb over the stile here on the right-of-way across the field belonging to the Nuadd farm but please keep to the river bank). These soils too are largely a uniform reddish-brown.

What does it all mean?

Soil forms over hundreds and thousands of years. Rain falls on the bare rock, till or river gravel (or whatever the parent material is at a particular place), plants grow and the finer particles and soluble minerals near the ground surface get washed downwards. Some end up in rivers and others are left behind in the lower parts of the soil. Meanwhile the remains of the plants growing – and dying – at the surface become mixed in at the top by the abundant creatures, such as earthworms, which live in the soil. Thus the top 10cm or so become more organic and we end up with a sequence of horizons from the organic surface A horizon, through the B horizon where material has accumulated from above and finally into the weathered surface of the parent material – the C horizon.

Down in the Senni valley the till and river sediments have been gradually changing in this way since they were first laid down. For the glacial till this was under the ice more than 12000 years ago while the river sediments are being actively redeposited now and may be no more than a few years or centuries old. Thus the valley soils show this simple progression from A to C horizon, although the strong red colour of the rock (and thus the parent materials formed from it) makes it hard to see the organic matter accumulated near the surface or the fine material redeposited in the B horizon, which are much clearer in soils elsewhere.

On the hills, however, the process has been a bit more complex and the result a little different. Site ●14 lies on soliflucted till, which contains few pores and through which rainwater flows only very slowly. Thus there has been less movement of fine matter downwards and the B horizon is less well-defined. The wetness of the soil, and the lack of pores, has also meant that it contains very little air and the few microscopic plants and animals which can live in such harsh conditions have removed such oxygen as was originally present and thus changed the minerals in the soil. The most visible effect of this has been to change the red iron oxide (the oxide of a form called Iron III) into a blue-grey form (Iron II). At the surface the organic matter, from dead plants and animals, have not decayed as much as they would in a dry soil and the result is a slightly darker, peaty A horizon.

Further up the hill at ●15, however, we cross from the till, which slid down the hillside by solifluction during the last glaciation, to the exposed rock it left behind. Here the ground is much better – though not perfectly – drained. The sequence of dark, light and brown bands are the result of two competing processes, waterlogging-weathering at the bottom and downward acid-solution above.

Look at the till exposed at ●16 and in sections further along the track nearby. You can see the remains of stones which have been so weathered that they are now soft enough to cut with a penknife. This weathering has happened because the water held in the soil has been gradually dissolving these stones for more than 10,000 years – a process accelerated here by the lack of oxygen in the soil when it is at its wettest. Here, and at ●14, you can see brown patches and mottles where a little oxygen, in water descending root pores, has allowed the iron to change back to its original Iron III form.

The black surface horizon at ●15 is an accumulation of organic matter, mixed into the surface soil minerals and preserved not so much by the wetness of the soil but by acidity. The living and decaying plants release weak acids, as does the rain, and this makes the surface soil inhospitable to earthworms and other plants and animals which would otherwise break the organic matter down. The lighter band below has formed because these same acids have dissolved the minerals which colour the soil – iron especially – and left behind only uncoloured, resistant minerals like quartz.

This kind of soil is called a Podzol (which means Ash Soil in Russian, since the bleached, light horizon looks a bit like wood-ash), the wet soil at ●14 (and in the wetter bits of the Senni valley bottom) is a Gley and the normal soils of the valley are called Brown Earths.

The multiple bands of colour seen at ●16 are the result of something rather different. Look at the hillside immediately above and you can see that the slope here has an unusual, stepped, disturbed appearance. These disturbances are the remains of drove-ways. The bands in the soil profile below are made up of soil which has been eroded from the slope above when the drove-ways became cut-up by the hooves of the animals. You can see that this happened in episodes of erosion and stability by the bands of eroded mineral soil and the more organic topsoil which has formed on top of them.

Returning to the valley itself, two recent bits of building work have exposed the soil profile and allowed us to compare soils formed in different materials. The construction of a new house close to the village hall ●19 created a 2 metre deep section through the soil. We found largely unweathered till at 1m from the surface – and we can tell it was unweathered because the stones within it had small cappings of fine, compacted silt which formed when the soil was frozen, perhaps 12-18,000 years ago, and which have been preserved ever since.

The section at Gorslwyd farm ●20 nearby looks similar but exposed the broken bedrock, rather than till, showing that the Senni glacier of the last glaciation had been grinding away at the rock but had not deposited much till here. The lack of bedrock at a depth of 2m at ●16 and nearby, but not at Gorslwyd, is further evidence that the village is built on a thicker mass of till, which might be a moraine

Resources: Geology

Perhaps the most useful guide for those wanting to know more about the geology of our valley is the volume “Geological Excursions in Powys” by N.H. Woodcock and M.G. Bassett (National Museum of Wales, 1993). This describes most of the rocks of the region as a sequence of themed chapters, each discussing the geology of a specific area – and usually of a specific period – as can be seen by following a route through the area. Chapter 14 describes the Devonian sequence and discusses the geology of the Senni bed exposures in detail.

Another excellent source is the short guide “Classic Landforms of the Brecon Beacons” by Richard Shakesby of Swansea University. This explains many of the more striking landforms of the Beacons, including some very close to Senni, and describes how they formed, with particular emphasis on the effects of the glaciations.

Finally, although it is not yet published, we eagerly await the release of the new British Geological Survey 1:50000 scale map of the area around Senni which was mapped in 2003-4. This will be accompanied by a monograph describing the geology of the area and presenting new evidence, especially about the Quaternary and Holocene – the glaciations and deposits laid down in this current interglacial period.