Sedimentary structures

I was intrigued by the 'forests' created in the sand by the rivulets of water draining from The Settlands beach near Little Haven, Pembrokeshire.

I was intrigued by the 'forests' created in the sand by the rivulets of water draining from The Settlands beach near Little Haven, Pembrokeshire.

Structural geology of Little Haven and The Settlands The cliffs from Little Haven to Broad Haven (and northward) display a spectacular range of geological structures, folding, faulting and thrusting, mainly in the Lower Coal Measures. The relatively weak mudstone and shale-dominated sequences show much incompetent deformation: tight, thrusted and overturned folds, in contrast to the thicker, stronger, sandstones which have formed relatively open and concentric folds. This is a close up view of the northern limb of the Fox Hole anticline taken from beach level. The style of folding is open and concentric, largely due to the thick (approx 70 m in total) and competent (strong) nature of the sandstone of which it is comprised. The sandstone displays a number of sedimentary features, including cross-bedding and channel erosion (visible just above and to the right of centre), formed by deposition in a high-energy delta distributary channel setting. Restoring the anticline to its 'unfolded' condition, these sedimentary structures then indicate a general current flow direction from south-west to north-east. Folding, faulting and thrusting make determination of the exact stratigraphic correlation of the Fox Hole sandstone difficult, but it is reasonably clear that this is the lowest stratigraphic unit in the Little Haven area and is well down in the Lower Coal Measures sequence.

Structural geology of Little Haven and The Settlands The cliffs from Little Haven to Broad Haven (and northward) display a spectacular range of geological structures, folding, faulting and thrusting, mainly in the Lower Coal Measures. The relatively weak mudstone and shale-dominated sequences show much incompetent deformation: tight, thrusted and overturned folds, in contrast to the thicker, stronger, sandstones which have formed relatively open and concentric folds. This is a view of the Fox Hole anticline taken from beach level. The style of folding is open and concentric, largely due to the competent (strong) nature of the sandstone of which it is comprised. A thin coal in the core of the anticline has largely been extracted by mining. The sandstone displays a number of sedimentary features, including cross-bedding and channel erosion, formed by deposition in a high-energy delta distributary channel setting. Restoring the anticline to its 'unfolded' condition, these sedimentary structures then indicate a general current flow direction from south-west to north-east. Folding, faulting and thrusting make determination of the exact stratigraphic correlation of the Fox Hole sandstone difficult, but it is reasonably clear that this is the lowest stratigraphic unit in the Little Haven area and is well down in the Lower Coal Measures sequence.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows a cross-section view of a locally derived beach boulder consisting of a channel lag deposit with ironstone pebbles and fragments set in a pale grey coarse sandstone matrix. The boulder is approx. 50 cm thick.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows the uoper surface of a locally derived beach boulder consisting of a channel lag deposit with ironstone pebbles and fragments set in a pale grey coarse sandstone matrix.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows planar laminated sandstones with an erosive base resting on the underlying sandstone unit. Planar laminated sandstones in this setting often indicate high energy, fast current flow. The photo is approx. 60 cm from top to bottom.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows planar laminated sandstones with an erosive base (marked by the walking stick handle) cutting into the underlying sandstone unit. Planar laminated sandstones in this setting often indicate high energy, fast current flow. The walking stick is 90 cm long.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows planar laminated sandstones with an erosive base (marked by the walking stick handle) cutting into the underlying sandstone unit. The walking stick is 90 cm long.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows laminated sandstones with two scoured channel bases cutting into the underlying channel-lag deposits. The walking stick is 90 cm long.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows a close-up view the erosive channel-lag deposits with a prominent layer (approx. 30 cm thick) of ironstone pebbles.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows a close-up view the erosive channel-lag deposits with a prominent layer of ironstone pebbles. The lens cap is 72 mm in diameter.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows the erosive channel-lag deposits with a prominent layer of ironstone pebbles. The lens cap is 72 mm in diameter.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows a close-up view of the erosive channel-lag deposits of sandstone blocks, ironstone pebbles and detatched coal lenses. The lens cap is 72 mm in diameter.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows erosive channel-lag deposits of sandstone blocks, ironstone pebbles and detatched coal lenses all cutting down at a steep angle into the underlying sediments. The walking stick is 90 cm long.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west. This photo shows erosive channel-lag deposits of sandstone blocks, ironstone pebbles and detatched coal lenses all cutting down at a steep angle into the underlying sediments. The walking stick is 90cm long.

Nolton Haven to Newgale The small Nolton-Newgale coalfield is separated by faulting from the main Pembrokeshire coalfield. It lies mostly within Pennant Sandstone Formation of the Upper Coal Measures rocks. The cliffs of Nolton Haven expose predominantly deltaic channel sandstones, often with highly erosive bases with coarse, rip-up, channel-lag deposits. The general dip is around 20° to the south-west.