Pembrokeshire geology

Saundersfoot to Swallowtree Bay geological structures Detail view no.9 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This photo shows the asymmetric syncline of the fold couplet pair. On the left, the fold limb is vertical, becoming overturned towards the top of the photo. The opposite limb dips gently to the south (right). The sandstone beds show channelling and thickness variations, especially just above the top of the walking stick. The sandstones also dispaly a strong fracture fan cleavage perpendicular to the bedding, in contrast to the darker, weaker, darker siltstones which show little or no cleavage. The walking stick is 0.9 m long. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detail view no.8 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This is virtually the same as detailed view no.6 but with the walking stick added for scale (0.9 m long) This photo shows the vertical limb of the fold couplet pair. In the centre, the vertical limb is composed mainly of sandstones with thin darker silty laminae. The sandstone beds show a strong fracture fan cleavage perpendicular to the bedding, especially marked on this vertical limb of the fold. Minor thrust planes disrupt the thinner sandstone beds by a few centimetres. To the left, finer, darker, laminated siltstones show little or no cleavage. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detail view no.7 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This photo shows the vertical limb of the fold couplet pair. In the centre, the vertical limb is composed mainly of sandstones with thin darker silty laminae. The sandstone beds show a strong fracture fan cleavage perpendicular to the bedding, especially marked on this vertical limb of the fold. Minor thrust planes disrupt the thinner sandstone beds by a few centimetres. To the left, finer, darker, laminated siltstones show little or no cleavage. I took this photo for the shapes, textures and colours as well as the geological interest, hence I omitted a scale object. The section shown is about 1 metre in height. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detail view no.6 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This photo mainly shows the vertical limb of the fold couplet pair. The axis of the anticline is on the left. In the centre, the vertical limb is composed mainly of sandstones with thin darker silty laminae. The sandstone beds show a strong fracture fan cleavage perpendicular to the bedding, especially marked on this vertical limb of the fold. I took this photo for the shapes, textures and colours as well as the geological interest, hence I omitted a scale object. The section shown is about 2 metres in height. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detail view no.5 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This is a view of the sandstones and laminated sandy siltstones on the southern limb of the anticline, close to beach level. The sandstone beds show thickness variation due to channelling and erosive bases into the underlying laminated sandy siltstone beds. The latter show fine-scale rhythmic fining-up laminae, which have slightly irregular slumped bases. All these sedimentary features demonstrate that these beds are the right way up! The lens cap is 72 mm in diameter. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detail view no.4 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This is a view of the siltstones and sandstones in the fold axis of the anticline below the small thrust which disrupts the fold axis. The sandstone beds show thickness variation due to channelling and erosive bases into the underlying laminated sandy siltstone beds. Apparent low-angle cross bedding in the central, darker laminated sandy siltstone bed may be partly due to dislocation and bedding plane slip at its junction with the next underlying sandstone. I took this photo for the shapes, textures and colours as well as the geological interest, hence I omitted a scale object. The section shown is about 1.6 metres in height. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detailed portrait format view no.3 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This is a view of the siltstones and sandstones in the fold axis of the anticline. The southern (left) limb of the fold shows much crumpling and shearing in the weaker, laminated siltstones. I took this photo for the shapes, textures and colours as well as the geological interest, hence I omitted a scale object. The section shown is about 1.8 metres in height. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detail view no.2 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. This is a view of the siltstones and sandstones in the fold axis of the anticline. The southern (left) limb of the fold shows much crumpling and shearing in the weaker, laminated siltstones. The stronger sandstone beds show a strong fracture fan cleavage perpendicular to the bedding, especially marked on the vertical limb of the fold. I took this photo for the shapes, textures and colours as well as the geological interest, hence I omitted a scale object. The section shown is about 1 metre in height. The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Detail view no.1 of the asymmetric anticline-syncline couplet in Lower Coal Measures sandstones and siltstones, just south of Swallowtree Bay. A curved, northerly dipping thrust just above the top of the walking stick disrupts the anticline fold axis. The thrust becomes near horizontal when followed up to the left of the centre, with crumpling and repetition of the bedding just above the thrust plane. The stronger sandstone beds show a strong fracture fan cleavage perpendicular to the bedding, especially marked on the vertical limb of the fold. Scale: the walking stick is 0.9 m long The photo of the entire structure is here:

Saundersfoot to Swallowtree Bay geological structures Just south of Swallowtree Bay near Saundersfoot, Lower Coal Measures sandstones and siltstones have been folded into an asymmetric anticline-syncline couplet. The folds display a variety of accommodation structures: minor thrusts disrupting the fold axes (particularly the antincline axis); minor thrusts causing crumpling and repetition of some bedding in the fold limbs; thinning of the fold limbs and thickenning in the fold axis. The sandstones display cross-bedding and erosive channel bases. Scale: the height of the exposed rock section is about 6 metres in the vertically bedded zone. Details of the fold couplet are shown in the photos which follow, indicted by the notes.

Saundersfoot to Swallowtree Bay geological structures Just south of the previous photo in Swallowtree Bay, Lower Coal Measures sandstones have been folded into a broad open syncline. The sandstones display cross-bedding and erosive channel bases.

Saundersfoot to Swallowtree Bay geological structures An asymmetric anticline chevron fold developed in Lower Coal Measures sandstones just north of Swallowtree Bay near Saundersfoot. The fold has a vertical to slightly overturned northern limb. In the centre of the photo, minor thrusts disrupt the fold axis slightly, to accommodate the 'space problem'.

Saundersfoot to Swallowtree Bay geological structures The Ladies Cave Anticline is a well-known and much visited structure in Lower Coal Measures rocks, just south of Saundersfoot harbour. This is a photo of the hinge zone in the 'cave', developed in the hinge zone of the fold. The weaker mudstones have 'flowed' up into the thickened fold hinge to form saddle structures. There are also small scale thrusts, marked by disruption of the brown ironstone layers. The horizontal ridges on the underside of the steep northern limb probably indicate development of a fan cleavage, although it does not appear to be present on the opposite limb. The main anticline photo is here:

Saundersfoot to Swallowtree Bay geological structures The Ladies Cave Anticline is a well-known and much visited structure in Lower Coal Measures rocks just south of Saundersfoot harbour. This is a fine example of a chevron fold, often developed in alternating layers of stronger and weaker rocks. The fold is asymmetric; the axis dips steeply to the south (left). Accommodation structures as a result of the 'space problem' include thinning of the fold limbs in the weaker mudstones layers, 'flowing' up into the thickened fold hinge to form saddle structures. There are also small scale thrusts in the hinge zone: Earlier photos here (September 2010) and here (February 1993)

Saundersfoot to Swallowtree Bay geological structures Detail of the faulted Lower Coal Measures rocks in the previous photo. I particularly liked the colours and textures of these rock and did not want to detract from these by including a scale object. The photo is about 1 metre top to bottom. The rocks are mostly grey mudstones with brown ironstone bands and nodules.

Saundersfoot to Swallowtree Bay geological structures Just south of Saundersfoot harbour, immediately adjacent to the previous photo, a set of extensional ('normal') faults with a small displacements cut Lower Coal Measures rocks: mostly mudstones with ironstone bands and nodules, with thin sandstone bands and laminae in the upper part. The small thrust with a footwall syncline in the previous photo is just visible at the lower right. The walking stick is 0.9 m long. Detailed part of photo here:

Saundersfoot to Swallowtree Bay geological structures Just south of Saundersfoot Harbour is a small thrust with a footwall syncline in Lower Coal Measures rocks. The anticline component of the fold couplet has been sheared out by the thrust. The rocks are mainly grey mudstones with brown ironstone bands and nodules, with thin pale grey sandy lamine in the upper part and to the bottom right. The walking stick is 0.9 m long.

Broad Haven (south) to Stackpole Head Viewed from just north of Stackpole Head, the narrow sea arches of 'Lattice Windows' form the headland at the south side of Barafundle Bay. In contrast to the gentle southerly dip of the Carboniferous Limestone at Stackpole Head, fold structures have resulted in an increasingly steep northerly dip at Lattice Windows.