Faults and Folds

When geologists talk about the structure of an area, they are referring to the pattern of faults and folds which affect the rocks.

Faults and folds come in all shapes and sizes.  There are for example complex faults within the Geopark which extend for dozens of miles across country and which include secondary faults and folds too.  These zones of intense upheaval are referred to in South Wales as ‘disturbances’.  On the other hand there are minor faults and folds which may only extend a distance of a few metres.

Faults and folds tend to form when the rock is put under great pressure.  During certain periods in the geological history of Fforest Fawr, landmasses have collided with each other through the process of continental drift.  At such times, beds of rock will buckle and crumple under the huge forces that are involved. These processes are particularly evident in the Silurian and Ordovician rocks of the Geopark.

Information about individual faults can be found here.

The Tawe Valley Disturbance is a zone of faulting and folding can be traced from Swansea all the way up the Tawe Valley through Ystradgynlais and Glyntawe into the central part of Fforest Fawr.  It continues northeastwards as the Cribarth and Swansea Valley Faults running just north of Brecon and on towards Hay on Wye.

The Neath Disturbance is a major set of fractures can be readily traced as it is responsible for the long straight course of the Vale of Neath.  It runs NNE through the Geopark from Pontneddfechan where it is spectacularly seen at Bwa Maen, beneath Llwyn-on Reservoir heading then towards Crickhowell.

The Carreg Cennen Disturbance is the most northerly of the three main lines of fracturing and folding to affect the Geopark, is traceable from the Tywi estuary past Carreg Cennen Castle and onward towards Trecastle and Sennybridge.  It continues for a very considerable distance to the northeast as the Church Stretton Fault Zone.

Other faults – there is a swarm of faults which are found running NNW to SSE through the Carboniferous rocks of the Geopark.  They are responsible for the alignment of certain valleys such as that of the Taf and play an important part in the ‘Waterfalls area’ around Ystradfellte and Pontneddfechan.

Most of the spectacular falls occur where these faults bring harder-wearing sandstones up against softer mudstones.