A trip to photograph an ancient weakness in the earths surface

My interest in the science of geology is its endeavour to unravel the history of the earth from rocks and structures, a complex story that requires the mind to grapple with the depth and enormity of geological time and processes. It is also personally rewarding to make a landscape photograph that conveys the geological story of a landscape.

From: Carney, J.N., Horak, J.M., Pharaoh, T.C., Gibbons, W., Wilson, D., Barclay, W.J., Bevins, R.E., Cope, J.C.W. & Ford, T.D., (2000), Precambrian Rocks of England and Wales, Geological Conservation Review Series, No. 20, Joint Nature Conservation Committee, 

At some point in the deep geological time of the Precambrian, the Cymru and Wrekin terranes collided and the collision zone (suture) or terrane boundary, has created an inherent weakness in the earths crust and a nucleation point for subsequent dislocations from external tectonic forces. Geological mapping has revealed that the Cymru/Wrekin terrane boundary has a long history of movement in the Precambrian, during the Caledonian Orogeny and then the Variscan Orogeny. These movements along the terrane boundary generate faults and folds (disturbances) in the overlying rocks and these features have been exploited by erosional processes, notably the actions of freeze-thaw weathering, flowing water and glacial ice. Which is a good thing, as the scenery of the Brecon Beacons National Park would be otherwise a lot photogenically poorer.

Faults of the Fforest Fawr Geopark
http://www.fforestfawrgeopark.org.uk/understanding/rocks-and-geology-of-the-geopark/faults-and-folds/geological-faults-in-the-geopark/

The head of the Senni valley has; a classic glaciated cirque; a U shaped profile; a miss fit water course of the Blaen Senni and the upper valley is aligned with the Tawe (Swansea) Valley/Cribarth disturbance. The valley orientation is initially NE before abruptly swinging north, which coincidentally aligns with another major structural trend which is termed Malvernoid. To the east of where the Senni valley swings north is the Cwm-du valley, a hanging valley above the floor of the Senni valley, with a suite of moraines and steep headwall of Graig Cwm-du. Cwm-du also has a mis fit stream and coincidentally the head waters of the Nant Cwm-du also parallel the Tawe Valley disturbance. This is an area I've visited on numerous occasions to try and make an image that I'm happy with.

There are a number of approaches to reach the vantage point on the Fan Frynych ridge over the Senni valley, the most scenic approach being through the Craig Cerrig Gleisiad National Nature Reserve and at this time of year, there is the potential for low angled sunlight on the northerly facing scarps and valley sides, with patches of heather in bloom for floral interest.

The weather forecast
Credit - Met Office

The forecast was optimistic considering the picture of the visible satellite, rainfall radar and wind strength readings from online weather station, nonetheless there was potential for some atmospheric lighting.

The scenic Craig Cerrig gleisiad
Foreground - vegetated antiscarps of the landslide and western headwall
Middle  - Craig cerrig-gleisiad
Horizon left - Cadair Arthur and shower cloud

Surveying the landscape to the south west from Fan Frynych's summit and it doesn't take much imagination to visualise a continuous plateau surface to Fan Llia, before erosion processes exploited  weaknesses in the bedrock and the dissection of the plateau. Sadly on this trip, as with previous trips the weather dictated the view of the Senni Valley would again remain unphotographed, this time the winds strength was problematic, maybe next time?

Undulating plateau dissected by Cwm-du

Cwm-du and the north facing scarp of Craig Cwm-du with layer cake stratigraphy of the Lower Old Red Devonian Senni beds

Instead a tactical retreat was made to a headwater stream of the Nant Cwm-du, with a view down into the Cwm-du and Senni valleys and 'relative' shelter from the wind. A patient wait for a break in the shower clouds, was spent donning and doffing rain wear and pondering plateau glaciation, cold based ice, periglacial processes, the thickness of soliflucted till, oh and the delights of descending through the landslide debris of Craig Cerrig Glesiad in head torch light ...

My patience was rewarded with a fortuitous break in the clouds. It wasn't the landscape image I had hoped for, but a satisfying one with the atmospheric lighting and a view along and down an ancient weakness in the earths crust revealed by erosion processes.