Hinge Zone

Disharmonic recumbent fold at Broad Haven - view of the hinge zone

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.

This photo shows a closer view of the hinge zone along the horizon of a thin coal seam and the immediately overlying dark shales, which have 'flowed' into the angular apex of the fold just to the left of centre. The relatively strong sandstone beds (orange-brown) have deformed into an angular chevron-shaped fold, while the softer grey seatearth below the coal has deformed into a rounded fold, seen in the lower part of the structure, to the right of centre.

The seatearth is also cut by a coarsely-developed axial planar cleavage (better seen in the previous photo), which is partly responsible for the 'nubbly' texture on the rounded part of the fold.