Introduction
The impressive sandstone gateway which forms the entrance to Astley Park was placed there around 1920, having formerly occupied the entrance to Gillibrand Hall.
A Grade II listed building, the gateway is described as:
Gateway arch, probably mid C19, formerly at Gillibrand Hall (q.v.), removed to this position as war memorial c.1920. Sandstone. Semicircular arch with very large rusticated voussoirs and pendent keystone, entablature, moulded cornice, parapet, and piers with tapered pilasters which have 3 fancifully vermiculated bands. Frieze now lettered:
PRO PATRIA 1914-1918
The main gate is flanked either side by two smaller gates mounted on sandstone gateposts.
In 2013 the arch underwent restoration work in the form of a £53,000 facelift as, while structurally sound, a survey found evidence of gradual deterioration due to weathering and water ingress.
Work included some stones being repaired and new stones carved to match the original, repairing the gates, painting them and rehanging them with new metal runners.
The top of the arch was lifted, repaired and capped to stop water getting in.
This timely restoration and preventative maintenance should help to ensure the structure stands up to the elements for at least another fifty years.
This EarthCache relates to the sandstone used to construct the arch, its formation and appearance and structures within it.
Logging Tasks
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Based on your study of the gateway arch from all sides and of the stone used to build it, and using the information on the cache page please tell me:
- What percentage of the stone blocks include oxides of iron?
- How many of the stone blocks are of one solid colour?
- Describe the ferruginous features that you see in this arch and where you see them.
- Thinking about how this rock formed, starting with loose sediments and becoming the solid rock you see today, classify its features, including those which are ferruginous, as depositional, diagenetic, deformational, or surficial.
- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
The features that characterise a sedimentary rock can form at diverse times and under very different conditions. Geologists divide these features into four classes:
- Depositional features (sometimes called primary features because they occur early on in the formation of sedimentary rocks) form while the sediment which the rock is made from is accumulating. They can tell much about the environments that the sediments came from and were deposited in.
- Diagenetic features (sometimes called secondary features) develop after sediment has accumulated and can include lithification - the transition from sediment to solid rock. Diagenetic features typically have chemical origins and include nodules and concretions.
- Deformational features result from the bending, buckling, or breaking of sedimentary layers by external forces.
- Surficial features develop in rocks at or near the surface where the rock is subject to groundwater percolation and the impacts of weather conditions. These features can reflect both physical and chemical changes.
Detrital sedimentary rocks, like this sandstone, are made up of rock fragments (sediments) held together by mineral cement.
In this sandstone the cementing minerals are oxides of iron. If you've ever seen rust, you've seen an iron oxide and you'll know that it's usually a shade of brown or orange or red. It should come as no surprise then that the presence of iron oxides in sandstone can be seen in varying shades / intensities of these colours, depending on the concentration levels of the oxides present.
Low levels of iron oxides typically result in sandstone with a pale tan colour. At the other end of the spectrum are sandstones with such high concentrations of iron oxides that they have a sometimes quite dramatic deep red colouration, which points to those sandstones being formed from sediments from a hot desert environment.
Variations in the concentration of iron oxides within a sandstone can give rise to patterns and to hard, round nodules which may be harder than the surrounding rock.
Ferruginous is a word used to describe substances which include oxides of iron in their makeup. Where iron oxides produce distinct features such as banding, rings and hard, round nodules, we might describe those features as being ferruginous features.
Time to look at some features common in sandstone...
Features common in sandstone
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Bedding planes - sedimentary rocks are made of fragments of other rocks called sediments, stuck together by mineral cement. Unsurprisingly, the sediments in sandstone are sand-sized grains - which makes them easy to see with the naked eye
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Because the sediments are laid down under the force of gravity they tend to settle in horizontal layers called beds. Beds are made up of sediments of roughly the same size and type. If the size and/or type of sediments being deposited changes distinctly a new bed is formed. The interface between beds is called a bedding plane. Sometimes the beds and bedding planes are easy to see, but sometimes they aren't so clearly visible.
The image to the right shows a section of sandstone in which the beds (layers) and bedding planes (darker bands between the layers) are clearly visible.
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Liesegang bands - occur naturally within many sedimentary rocks - like sandstone for instance. They can occur in igneous and metamorphic rocks too but this is far less common.
The most basic form is one of parallel bands of oxide material which tend to vary between light and dark and for this reason Liesegang bands are often misinterpreted as beds / bedding planes but one of the characteristics of these bands is that they ignore existing bedding - but they often concentrate / intensify around joints and cracks in the rock.
In the image on the right the ribbon of narrow bands remain parallel to one another while forming flowing curves which bear no correlation to the rock's original bedding.
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Liesegang rings - it should come as no surprise at all by this point that Liesegang rings are just Liesegang bands which form an enclosed loop (or ring, obviously
).
Rings may form as nice, regular, concentric circles but that needn't be the case, rings of all sorts of shapes can occur. Where the rings are circular though they can easily resemble tree rings in appearance and we might even be forgiven for thinking we're looking at fossilised wood rather than rock 
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We might also be forgiven for thinking, when presented with concentric rings and knowing that rocks form in layers, that we're looking at circular layers of rock which somehow grew out from a central point - but that's not the case as we know that the rings developed long after the layers of sediment which make up the rock were laid down horizontally.
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And so not to bed - Sediments and sedimentary rocks are characterised by bedding, which occurs when layers of sediment, with different particle sizes are deposited on top of each other. These beds range from millimetres to centimetres thick and can even go to metres or multiple metres thick - which is one reason why it's not always easy to see the bedding in the relatively small scale pieces of rock used to construct buildings.
The image to the right though does clearly show a mostly vertical Liesegang band cutting across all of the horizontal beds in the stone - just so you know what it looks like . It's not the best photograph in the world but the spot I took it in was fairly shady.
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Complexity - So far the examples given have focused on simple forms of Liesegang patterns but the phenomena is not limited to simplicity and can in fact result in considerable complexity in terms of shapes, sizes, colour variation and the subsequent impacts on the rock surfaces of weathering and erosion.
Darker bands indicate higher concentrations of iron oxides which in turn make those parts of the rock harder than areas with lower oxide concentrations. This often results in the surface of the rock being sculpted by weathering and erosion as the softer parts are worn away while the harder parts are left behind, which can turn a flat stone face into a miniature 3D landscape.
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Honeycomb weathering - is common in coarse-grained sedimentary rocks and also in plutonic igneous rocks such as granite.
While the appearance speaks for itself, the possible causes put forward by members of the scientific community are many and varied - although there does seem to be a general concensus that it is a phenomena associated with environments with high salt concentrations.
It is also possible that the concentration and type(s) of cementing minerals holding the sediments together in a body of rock result in variations in the rock's resistance to the agents of weathering, such that different parts weather at different rates.
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Nodules / Concretions - After considerable research and expert consultation the consensus is that the terms nodule and concretion are used by geologists interchangeably, and their definitions are blurred - so I'm just going to refer to them as nodules.
Nodules represent a gathering/concentration of minerals in one place within a parent rock, forming a structure which is commonly rounded and harder than the surrounding rock and thus more resistant to weathering. Their greater resistance to weathering often sees them standing proud of the rock they are embedded in and, in cases of severe weathering of the rock, nodules may be weathered out completely, leaving behind a hollow depression.
Iron nodules are common, often spheroid in nature and thus easy to spot because they look a lot like rusted ball bearings.
As shown in the image to the right - nodules may be solid but are often hollow, sometimes because material originally at their core has weathered out or fallen out when the parent nodule was broken open.
If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there .
Please submit your logging task responses before posting your log.