Along the crest

Disharmonic recumbent fold at Broad Haven - view along the fold crest.

On the north headland of Broad Haven, Pembrokeshire, is this spectacular fold structure in Coal Measures strata. The fold is essentially an anticline strongly overturned northwards (to the left) so that its fold axis is near to horizontal.

The relatively strong sandstone beds (orange-brown) have deformed into an angular chevron-shaped fold, while the softer seatearth below the coal has deformed into a rounded fold, seen in the lower part of the structure, almost from the camera viewpoint. This has been achieved by considerable slip along individual bedding planes and softer, incompetent, material has been squeezed into the hinge zones from the fold limbs. This has occurred particularly along the horizon of a thin coal seam and the immediately overlying dark shales, which have 'flowed' into the angular apex of the fold to the left of centre.

The seatearth is also cut by a coarsely-developed axial planar cleavage, which is partly responsible for the 'nubbly' texture on the rounded part of the fold. The cleavage fans out almost radially, best seen in section on the rounded bed in the middle distance.