Concretions in sandstones of the Inner Hebrides, Scotland

Mark Wilkinson (UK)

Concretions are a common feature in many sedimentary rocks, yet they seem sometimes to be misunderstood. So, how do concretions form? As well-studied examples, let’s look at the ones found in some of the sandstones of the Scottish Inner Hebrides, notably the islands of Eigg and Skye. The concretions are found in several formations, but perhaps the largest and most spectacular are in the Valtos Sandstone Formation of the Great Estuarine Group. This was originally named the Concretionary Sandstone Series after the prominent metre-scale concretions. It is Bathonian in age (Middle Jurassic) and is interpreted as having been deposited in a coastal environment. The Great Estuarine Group is becoming famous for its abundant dinosaur footprints and much rarer skeletal material.

The concretions themselves vary from spherical to elongate volumes of rock and are typically from around 50cm to one metre or more in diameter. They are also often coalesced into groups (Fig. 1). Inside the concretions, the spaces between the sand grains are filled completely with a calcite cement. The concretions are resistant to weathering compared to the host sandstone, which is fairly soft, so stick out from the cliff in a sometimes rather alarming manner as you walk below them. I’ve been visiting the concretions sporadically for around 30 years and some of the ones that I photographed in the cliffs in the 1980s are now lying loose on the beach. None of them have fallen while I’ve been there, touch wood.

How did the concretion form?

There is general agreement that, if you were to time-travel back to the Hebrides in the Jurassic when the sediments were being deposited, you would not have seen any concretions forming on the sediment surfaces. You could have roamed the sandy beaches or have swam in the river channels, lagoons and shallow seas, and seen no sign of them. You might have seen sauropod dinosaurs in the lagoons and carnivorous reptiles in the sea, so your tour would have been interesting but not because of concretions sticking out of the floor. What you would have seen, however, were large numbers of shells scattered about, especially (in the case of the Valtos Sandstone) the bivalve, Neomiodon. Some of the Neomiodon were present as shell banks, which we now find as limestones; others were present in the sandier sediments.

These shells turn out to be important, as the shell of Neomiodon was made of the calcium carbonate polymorph, aragonite. Aragonite, at typical Earth surface temperatures and pressures, is less stable than the better-known polymorph calcite. As the shell-rich sands were buried beneath layer after layer of younger sediment, so the unstable aragonite dissolved into the porewaters and reprecipitated as the more stable calcite – as concretions. Each concretion started growth from a point and expanded outwards, filling the porosity that was originally present between the sands grains and enclosing the sand grains in what is known as poikilotopic texture.

A minority of broken-open concretions have a feint concentric colour banding (in shades of grey-brown) that fits with this mode of growth. The calcite crystals themselves are up to several centimetres long and can be seen on broken surfaces when they catch whatever sunlight is around. In a rare example of humour in the scientific literature, the original description of this feature (by Julian Andrews and John Hudson, who both worked extensively in this area) includes a comment that sunny days in the Hebrides are more common than many people think. This fits with my personal experience, as for my first field season of my PhD, I arrived around Easter time with lots of warm and waterproof clothing. All I really needed, it transpired, was a sunhat and a really big bottle of sun cream.

At what depth did they form?

One question that we can ask about concretions concerns the depth at which they form, that is, how deeply buried beneath the surface were they when they grew? Did they start to grow while only a few centimetres below the sediment surface or are they later, perhaps growing when buried tens or hundreds of metres deep, or even kilometres below the surface? We can partly answer this question using observations that anyone can make in the field. One piece of evidence is that there are no (known) concretions that were eroded out of their host sandstone in the Jurassic and subsequently redeposited and buried in a new location. If we could find such a specimen, it would show that at least some of the concretions formed within reach of surface erosion – perhaps a few metres deep, as a river channel migrated laterally across a delta top for example, or as a storm eroded the sea floor. You might recognise such a concretion as the bedding seen within the concretion could be tilted – compare this to Fig. 2, where the feint bedding visible within the concretion is clearly horizontal. Or perhaps the concretion would have a covering of encrusting organisms, such as serpulid worms or oysters, from the time it spent on the surface.

We also know that the Hebridean concretions formed before the sediments reached their maximum depth of burial. As the sandstones were buried, so the weight of the accumulating sediments compacted the sandstones, pushing the sand grains closer together and reducing both the porosity of the sediment and the sediment thickness. However, the concretions do not have porosity as the spaces between the sand grains are filled with incompressible calcite – so the sandstones did not compact. This resulted in the bedding of the sandstones being distorted around the concretions – not by much, but enough to see (Fig. 2).

I once measured the compaction around ten concretions from the Valtos Sandstone Formation on the Isle of Skye and found that the sediments were compacted by around 20% more than the concretion. It turns out that it is virtually impossible to turn this number into an actual depth at which the concretions formed, except to say that is was ‘shallow’. It doesn’t seem to be possible to say whether the compaction was the result of the weight of overlying sediment of Jurassic or perhaps Cretaceous age, or of the overlying flood basalts of the Paleocene that are associated with the opening of the North Atlantic.

These lavas are well known to mineral collectors, as circulating fluids within the lava pile precipitated zeolite minerals in cavities and cracks within the basalts. The lavas are equally well known to tourists (even if they have little idea of what they are looking at), as they form the dramatic land-slipped scenery of the Trotternish Peninsula on Skye (Fig. 3) that was used as background scenery in the Hollywood film ‘Stardust’.

Why and how did they form?

This much is well understood about concretions. However, it is easy to ask questions that are tricky to answer and interesting to ponder while in the field, or while relaxing after a hard day’s fieldwork. Such questions include why a concretion grew in a particular location – why not a few centimetres or metres to one side? To some extent this is a bit like asking why an individual rain drop falls where it does, but sometimes clues can be found that suggest an answer. In some sediments, concretions commonly have objects at their centres – fossils for example, such as pieces of wood or ammonites, as anyone who has been to Whitby knows.

With very large concretions such as the Hebridean ones, you cannot crack open the concretions with a hammer to see what is there (and the coast is a SSSI), and the chances of finding a concretion that is naturally split exactly down the middle are not high. Personally, I’ve never seen any obvious object in the centre of a sandstone concretion from the Hebrides, though as shells are fairly common, perhaps one of these might have acted as a nucleus for growth. However, why any single shell should be better than the countless others is equally challenging to answer.

One field observation with no published explanation is the weathered out ‘notch’ that partly surrounds many of the Hebridean concretions (Figs. 2 and 4). Presumably, the sands around the concretions were more easily eroded than those further away – but why? Was there a mineral cement that prevented the rock from compacting that has weathered away leaving easily-eroded sandstone? If so, what was it – unfortunately it’s tricky to study something that isn’t there anymore. I’ve seen similar weathered notches around other concretions, for example, in Carboniferous sandstones on the Island of Arran in Scotland, so whatever the explanation, it has to be applicable to other locations than just the Hebrides. I’m now inclined to think that the ‘notches’ were formed when the supply of aragonitic shells ran out.

The concretions probably grew in slowly flowing groundwater, which, while there are shells present, would have slowly dissolved the shells and hence been saturated with respect to calcite or aragonite (depending on what the shells were made of). Eventually, however, there would have been no more shells left in the sandstone between the concretions, but groundwater would have continued to seep into the sandstones and flow past the concretions. As this groundwater would have passed through a soil to make its way into the subsurface, it would have picked up carbon dioxide and probably natural carboxylic acids, making it acidic. This water would have been capable of dissolving the concretions, starting, naturally, at the outer edges.

So, perhaps surprisingly, the concretions may have started to dissolve pretty soon after they stopped growing. As an alternative, perhaps the concretions didn’t start to dissolve until the heat of the Paleocene lavas and the intrusive centre of the nearby Cuillin Mountains of Skye started to mobilise groundwater. Regardless of when it occurred, the sand grains left exposed as the calcite was removed would never have been compacted (as they were previously protected by the calcite), so would have been only poorly consolidated and hence easily eroded by modern weathering, which has resulted in the notches around the concretions.

Despite the difficult-to-answer questions that we can ask ourselves about concretions, we know fundamentally how they form – after the host sediment has been buried below the Earth’s surface, as a diagenetic reaction as the sediment consolidates into rock. Unfortunately, this has not been universally understood. The excellent ‘Geology and Landscapes of Scotland’ by Con Gillen has a photograph of a large concretion in Jurassic sandstones on the Isle of Eigg, which is labelled as an algal growth on the Jurassic sea floor.

Obviously. opinions vary about geological matters, but I’m pretty sure the Eigg concretions are sub-surface, diagenetic features. It should be said that there are algal beds in the Great Estuarine Group – they just aren’t concretionary. Concretions are important from a scientific perspective, as they form a record of the conditions in the sediment as it was buried, such as the chemistry of the porewater. It has been worked out that the Skye concretions formed in a lens of freshwater that extended out under the sea from the contemporary land. It has even been worked out how long a single concretion took to grow: between five to ten million years for a one metre diameter specimen in the Valtos Sandstone, so watching one grow wouldn’t have been a spectator sport.

The concretions have also proved useful for preserving vertebrate fossils – the best skeletal remains are preserved ‘in the round’, that is, in full 3D, inside concretions. Otherwise, bone isn’t especially strong when pitted against the weight of hundreds of metres of overlying sediment and can be badly crushed. The difficulty then becomes extracting the fossils from the concretions, which typically involves a rock saw and some ingenuity (as well as a permit for collecting).

Fortunately for us, we have the help of local builder Dugald Ross, who also owns the fantastic small museum at Ellishadder, near Staffin on Skye (http://borve.net/staffin-museum.co.uk/). This houses an amazing collection of footprints and fossils, and shouldn’t be missed if you are on the island. Finally, the concretions are of interest to the oil industry, as, when similar examples are found in oil reservoirs, they can make getting the oil out of the ground a good deal trickier. Studying well-exposed examples, such as the Hebridean ones, can help us to understand concretions in deeply buried reservoirs such as those under the North Sea.

As a final point, the concretions we find in sandstones are really not very similar to the concretions found in mudrocks such as shales. The latter form by different chemical processes, but still during shallow burial – we don’t find them sticking out of muddy modern sediment, though plenty of people have looked.

About the author

Mark Wilkinson worked on the geochemistry of the concretions of the Hebrides for his PhD. He is now a Senior Lecturer in the University of Edinburgh, Scotland, and is still working on the geology of Skye and the other islands, but is now looking for dinosaurs and the environments that they lived in. Mark directs the University’s MSc in GeoEnergy.