Columnar jointing in dolerite, Garn Fawr, Mynydd Dinas

The location of the columnar jointed "plug" on the lower Garn Fawr tor.

Polygonal jointing revealed on an ice-smoothed surface towards the base of the exposures.

On the western extension of the Garn Fawr tor, on Mynydd Dinas, I spotted something I have missed on all my previous visits -- a large "plug" of dolerite with columnar jointing, standing quite prominently above the surrounding dolerite which has much more widely spaced jointing.  The plug is approx 10m x 10m in extent, and we can see a total depth of almost 30m down to the grassy slope at the base of the tor.  the columns are tightly-packed an somewhat irregular, with a dip towards the SE.   The polygonal jointing pattern is assisting in the breakdown of the plug, which must at one time have been much more prominent.  Around and below the plug there is a litter of small broken blocks, giving the appearance of scree -- this is very different to the character of the slopes around the rest of the tor.  On the lower slope there are various ice-smoothed surfaces which are perpendicular to the dip direction of the columns, thus displaying the polygonal jointing perfectly.

This phenomenon is relatively rare in the Preseli uplands.  Parker Pearson has claimed that there is columnar jointing on Carn Goedog, and Darvill and Wainwright have claimed that it exists on Carn Meini as well.  But I disagree with all of them; on those tors there are certainly occasional elongated "pillars" created by the existence of parallel joints, but they are random rather than organized in the manner of the Garn Fawr plug. They do not have true polygonal cross-sections like those illustrated below.

The broken surface at the top of the plug.  The terminated columns of dolerite are 

dipping towards the SE.

Columns exposed at the base of the cliff.

Here the joints are being forced apart, leading to the eventual collapse of column fragments

onto the slope beneath.