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.
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