Folding at Monkstone Point, near Saundersfoot, Pembrokeshire

Originally uploaded for the Guesswhere UK group.

Complex folding in Namurian rocks at Monkstone Point near Saundersfoot, Pembrokeshire, in the bottom left-hand corner of Wales.

This is a complex anticlinal chevron fold. The main limbs of the fold are the straight parts, dipping at 40° to the south (L) and 30° to the north (R).

One feature of tight folds like this is the SPACE problem. The rocks in the central part of the fold get compressed as the available volume they have to occupy reduces as the folding progresses. This leads to all sorts of seemingly chaotic deformation of the stuff in the middle. In this instance, the core of the fold is comprised of thin beds of sandstone interleaved with thin soft shale layers. The latter especially has deformed almost plasticly under the compressional stresses, and the thin sandstones also to a lesser extent. This results in the incompetent deformation structures seen in the photo, to accommodate the reduced space available in the core of the fold.