Sedimentary structures

This is the classic exposure of the Pipe Rock Member of the Eriboll Formation (early Cambrian) at Skiag Bridge, Loch Assynt, Sutherland, Scotland. Cross-bedded pale grey quartzitic/feldspathic marine sandstones deposited as sand bars in a near-shoreline environment are stained purplish-red and penetrated by Skolithos burrows infilled with creamy grey sandstone. This indicates that there was periodic halting of sedimentation and near, or actual emergence, so that oxidation of the iron and manganese minerals in the sediments could occur and the top part of the sediments colonised by burrowing organisms, probably worm-like animals.

This is the classic exposure of the Pipe Rock Member of the Eriboll Formation (early Cambrian) at Skiag Bridge, Loch Assynt, Sutherland, Scotland. Cross-bedded pale grey quartzitic/feldspathic marine sandstones deposited as sand bars in a near-shoreline environment are stained purplish-red and penetrated by Skolithos burrows infilled with creamy grey sandstone. This indicates that there was periodic halting of sedimentation and near, or actual emergence, so that oxidation of the iron and manganese minerals in the sediments could occur and the top part of the sediments colonised by burrowing organisms, probably worm-like animals.

This is the classic exposure of the Pipe Rock Member of the Eriboll Formation (early Cambrian) at Skiag Bridge, Loch Assynt, Sutherland, Scotland. Cross-bedded pale grey quartzitic/feldspathic marine sandstones deposited as sand bars in a near-shoreline environment are stained purplish-red and penetrated by Skolithos burrows infilled with creamy grey sandstone. This indicates that there was periodic halting of sedimentation and near, or actual emergence, so that oxidation of the iron and manganese minerals in the sediments could occur and the top part of the sediments colonised by burrowing organisms, probably worm-like animals.

The Buck Stone is a remnant edge tor comprised of the Quartz Conglomerate - the basal part of the upper Old Red Sandstone (Devonian). This is the more usual view of the Buck Stone. A more detailed view of the rock surfaces is here: www.ipernity.com/doc/earthwatcher/39024128 Although in located in Gloucestershire, England, the Welsh border is very close by, at the bottom of the slope. The fields in the middle distance and the far landscape are entirely in Wales.

The Buck Stone is a remnant edge tor comprised of the Quartz Conglomerate - the basal part of the upper Old Red Sandstone (Devonian). These cross-bedded sediments were deposited in alluvial fan or flash flood deposits, mostly in high-energy rivers draining an arid, mountainous continental area. There are many quartz pebbles, hence the name, but also rock fragments and more exotic pebbles such as jasper, all set in a medium - coarse sand matrix. Although in located in Gloucestershire, England, the Welsh border is very close by, at the bottom of the slope.

A day on Quinag - photo 14 . This is the final south-west ridge leading to Sàil Gharbh (808m), the highest summit of Quinag in the NW Highlands of Sutherland. The pink rock is Torridonian Sandstone (Applecross Formation, Precambrian), overlain unconformably by the light grey Basal Cambrian quartzite of the Eriboll Formation.

On the limb of an anticline, near-vertically bedded turbidite sandstones in the Bude Formation (upper Carboniferous) display sedimentary structures which can be used to determine the 'way up' of the bedding. Here, the sandstones display small-scale slump or load structures on the basal surfaces, and some beds show 'fining up' on the grain size (indicated by gradual darkening of the bed). These structures demonstrate that the vertical beds here get younger to the right. Here's structure in its proper setting and orientation: www.ipernity.com/doc/earthwatcher/39024212 and here: www.ipernity.com/doc/earthwatcher/39024214

Turbidite sandstones in the Bude Formation (upper Carboniferous) display sedimentary structures which can be used to determine the 'way up' of the bedding. Here, the sandstones display small-scale slump or load structures on the basal surfaces, and some beds show 'fining up' on the grain size (indicated by gradual darkening of the bed). These structures demonstrate that the beds here are the right way up, i.e. have not been completely inverted by the strong folding this area has undergone. The sequence shown here gets younger upwards and to the right.

Small normal faults in the Triassic age Budleigh Salterton Pebble Beds and the overlying Otter Sandstone. The Pebble Beds were deposited in desert conditions by intermittent torrential floods, perhaps in a wadi type of environment. The Otter Sandstone is a fluvial sand deposited by an extensive river network flowing through low lying desert conditions. The prominent honeycomb weathering is due primarily to modern wind erosion. Location: West Cliff, Budleigh Salterton, Devon. A photo of the same location taken when I was a student 40 years ago (scary!) is here: www.ipernity.com/doc/earthwatcher/39022690 Notice how there is much more debris banked up against the cliff now, compared with then.

The Budleigh Salterton Pebble Beds (Triassic). These rocks were deposited in desert conditions by intermittent torrential floods, perhaps in a wadi type of environment. There is a crude depositional dip of about 20° to the east (right) visible just above the grass-covered debris. The main part of the exposure is a generally fining-up sequence with a single sand layer visible in the upper part, which could represent a single flash flood event. Location: West Cliff, Budleigh Salterton, Devon.

The Otter Sandstone (Triassic) is a fluvial sand deposited by an extensive river network flowing through low lying desert conditions. The prominent honeycomb weathering is due primarily to modern wind erosion. Location: West Cliff, Budleigh Salterton, Devon.

A southerly view over Corporation Marshes towards Dunwich from Walberswick beach. There is not much separating the sea from the marshes. At times of high spring tides coinciding with a tidal surge down the North Sea, this area is flooded by the sea. This whole area is a Nature Reserve, home of bittern, marsh harrier and avocet, among others. However, the Environment Agency plans to discontinue maintaining the shingle bank sea defence and let the area revert to estuary. A depression with its associated warm from is rapidly moving in from the west. It was raining within three hours of this photo being taken. I love the wild bleakness of this stretch of coast and its big skies. This photo is a panorama consisting of 5 frames stitched together.

This is the Duckmanton Railway Cutting Site of Special Scientific Interest, near Chesterfield, north-east Derbyshire. In the lower part of the section is the Chavery Coal, overlain by shaly mudstones containing non-marine bivalves and ostracods. This exposure of the Chavery coal was worked (illegally) by desperate miners during the 1984-85 miners' strike. (Chris Darmon, pers. comm.) The railway cutting is a site of international geological importance, as it contains the stratotype section for the Anthracoceras vanderbeckei (Clay Cross) marine band which is the boundary between the Langsettian and Duckmantian Stages of the Carboniferous, informally the junction between the Lower and Middle Coal Measures. The site is managed by the Derbyshire Wildlife Trust and is accessible by permit only. www.derbyshirewildlifetrust.org.uk/reserves/duckmanton-railway-cutting

A view of the strata exposed in the steel arch shelter in the Duckmanton Railway Cutting, near Chesterfield, north east Derbyshire. The Chavery Coal is visible in the lower left. This is overlain by shaly mudstones containing ironstone bands and nodules. Fossils in the roof mudstones include non-marine bivalves ( Anthracosia ) and and ostracods ( Geisina ). The railway cutting is a site of international geological importance, as it contains the stratotype section for the Anthracoceras vanderbeckei (Clay Cross) marine band which is the boundary between the Langsettian and Duckmantian Stages of the Carboniferous, informally the junction between the Lower and Middle Coal Measures. The site is managed by the Derbyshire Wildlife Trust and is accessible by permit only. www.derbyshirewildlifetrust.org.uk/reserves/duckmanton-railway-cutting

This vertical slab of sandstone is at Northcott Mouth, near Bude, north Cornwall. See geotag for exact location. This is part of the Bude Formation (upper Carboniferous) - deltaic and some turbidite sandstones interbedded with shales. The sandstones are relatively thick, compared with the underlying Crackington Formation, and as a result, more competent. The response to the Variscan earth movements at the end of the Carboniferous was to form more open, mostly upright or slightly asymmetric angular folds, compared with the cascades of tight recumbent folds which characterise the Crackington Formation a few kilometres to the south. This slab is the part of the vertical northern limb of an asymmetric anticline, and the view is of the underside of he bedding. There are sole structures - mostly trains of ripples/scours - on this surface, proving that it is the base of the bed. I would guess that the local rock climbing fraternity have given this slab a name, so if anyone knows it please let me know. Thanks!

Spectacular sole structures - scours, ripples, grooves, etc. - on the inverted (upside-down) base of a turbidite sandstone slab in the Crackington Formation at Little Barton Strand, near Crackington Haven, north Cornwall. The palaeo-current direction in this photo was from bottom to top, which on restoration of the slab to its original non-inverted state, corresponds to a palaeo-current and a sediment derivation from the south. A full explanation of the structures and what they can tell us can be found here: www.ipernity.com/doc/earthwatcher/39023846

Another view of the sole structures on the inverted (upside-down) base of a turbidite sandstone slab in the Crackington Formation on the south side of the beach at its type locality, Crackington Haven, north Cornwall. The palaeo-current direction in this photo was from lower left to upper right. The scale can be deduced from the limpets - the largest is about 4 cm across. A full explanation of the structures and what they can tell us can be found here: www.ipernity.com/doc/earthwatcher/39023846

These elongated ridges are seen on the inverted bedding surface of a turbidite sandstone bed (Crackington Formation, upper Carboniferous) on the south side of Crackington Haven, north Cornwall. The scale can be deduced from the limpets - the largest is about 4 cm across. The ridges are casts of sandstone infill into what was originally a hollow groove in the underlying sediment. The grooves were formed at the time of sediment deposition by fast-flowing water making small erosion scours around irregularities on the Carboniferous sea bed. Sometimes the current flow was sufficiently powerful to drag along pebbles and small boulders, which created long linear grooves. Here, the rocks have been turned almost completely upside-down by intense folding at the end of the Carboniferous, so the grooves now appear as ridges. These sedimentary structures are generally known as 'sole structures' and are valuable as 'way-up' indicators - we can tell whether the rocks are the right way up or have been inverted, as is the case here. Also in this example, the scours are asymmetrical - deeper at the upstream end (lower right) and shallowing off at the downstream end (upper left), so they also provide evidence for the water current direction at the time of deposition. So in this case, the water flowed from lower right to upper left. But the rocks have been inverted by folding, so..... Here, the rocks are dipping (sloping) to the north (right) at about 30°, having been completely overturned by the folding. To restore this slab into its pre-folding position we need to imagine it 'flipped over', by rotating it through approx. 150° in a clockwise sense, like closing the left-hand side of an open book. Once we do this, the sole structure ridges on the upper surface of the slab become protrusions on the underside of the slab, with the asymmetric deeper upstream side to the left (south) and the shallower downstream side to the right (north). Therefore, the original current direction which transported the sediment of the Crackington Formation was from south to north: the source of the sediment lay to the south, and was transported northwards by the water currents. NB: The information board by the public toilets at Crackington Haven is excellent, except for where it states that the Crackington Formation sediment derivation was from the north. Sorry, but they've got that bit wrong! The evidence is there on the beach for all to see.