This is an urban EarthCache in which geocachers are invited to examine a sundial in Cambridge city centre. The learning point of this EarthCache is to get the geocacher to become familiar with sedimentary rock, in particular Bath Stone.
Everything you need to answer the questions is available by attending the co-ordinates and reading this lesson. I don’t anticipate you will have to research anything extra online, although you’re welcome to do so if you wish to.
Keywords for this lesson:
precipitate - the process that separates solids from a solution
Jurassic - a geologic period that spanned 56 million years, starting from the end of the Triassic Period 201.3 million years ago until the beginning of the Cretaceous Period, 145 million years ago.
comminuted - reduced to minute particles or fragments.
subjacent - situated below something else.
The Pembroke College sundial is a dial is on the wall of Foundress Court. The original proposal for this large wall sundial came from Eric Parry, the architect of the Foundress Court Building. The diallist was Frank King and the overall design was by Lida Cardozo Kindersley of the Cardozo Kindersley Workshop. Most of the stone cutting was undertaken by Helmut Hochrein. He cut the main dial in 1997 and the Equation of Time Panel in 1998.
The principal dial furniture consists of hour-lines marked from 6 am to 4 pm and three constant-declination curves (a pair of hyperbolic arcs for the two solstices and a straight line, called the equinoctial line, for the equinoxes).
This sundial has a gnomon and a nodus, both made of stainless steel. Time is determined by noting where the shadow of the gnomon falls among the hour-lines and the declination of the sun can be estimated by noting where the shadow of the nodus falls among the constant-declination curves. The sundial won a commendation from the British Sundial Society.
Since this is an urban EarthCache in the middle of Cambridge city centre, there are no cliffs, quarries or topographical features in the vicinity that can reveal the significance of where the stone comes from or how it was formed. However, by looking at the rock itself, we can draw certain inferences about the conditions it formed in. A more detailed look at the sundial will give some insight to it's provenance (the kind of environment that produced the sand.)
First, let’s look at rocks themselves, and then in particular limestone:
Introduction to rocks
Minerals make up rocks. Rocks are formed in many different types of environment. These can be on, or within the Earth's crust. There are three types of rock, and each is formed in a different way:
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma (molten rock.)
Metamorphic rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks.
Sedimentary rocks are formed on the Earth’s surface from the products of weathering which then becomes cemented or deposited. Limestone is an example of a sedimentary rock.
All three types of rock make up the Earth’s lithosphere, the outermost layer. The lithosphere averages about 100 kilometres in thickness.
Oolitic Limestone
The great majority of the limestones that are found in monuments, buildings and sculptures formed millions of years ago on the floor of ancient shallow seas. Limestone can be divided into oolitic or non-oolitic limestones. Some people think that the term ‘oolitic limestone’ covers all limestones from the Jurassic limestone belt, but actually it only refers to the specific geological character of some of them.
Limestone is formed when sediment gathered on the floor of these ancient seabeds. It principally consisted of clean-washed sand-sized grains of calcium carbonate. Some of these grains came from the shells and calcified skeletons of sea creatures. Others grew there, on the seabed, as the mineral calcite was precipitated around smaller particles (in the same way that limescale might build up on the inside of a kettle.) All limestones have over 50 per cent calcium carbonate and true limestones have over 90 per cent calcite.
This second category constitutes the ooliths. Typically, they measure between 0.2-0.8mm in diameter. This is because once they had reached the maximum size they abraded away as fast as they grew, and so did not grow any bigger. They are also characteristically very well rounded or spherical. When sliced through and viewed under a microscope, the layered structure of the precipitated calcite around the central nucleus is seen. They appear as spherical grains composed of concentric layers. Strictly speaking, oolites consist of ooids of a diameter between 0.25–2 mm. Rocks composed of ooids larger than 2mm are called pistolites.
Many Jurassic limestones consist mostly of these ooliths, sometimes containing a fossil or fragment. Others have much more shell debris in them, either scattered through a mass of ooliths or more usually in debris-rich layers that alternate with oolith-rich layers. This pattern reflects the surging of currents during the initial accumulation of the sediment and is very evident in limestones from near Bath.
Other limestones accumulated on the seafloor in areas where ooliths did not form, so the resulting limestones only contain shell fragments from the invertebrate fauna that originally lived there. Geologists refer to these as bioclastic limestones.
Because the smooth spherical character, size-range, and abundance of ooliths is highly distinctive, there should be no problem in recognising them on a cut or rubbed stone surface. This kind of oolitic limestone is visually quite distinct from a limestone that is principally composed of comminuted shell debris.
However, even amongst oolitic limestones, sometimes they can look quite different to each other. This is due to the disposition of the natural mineral cement within the stone. Limestones acquire their hardness by growth, during burial, of calcite crystals in the tiny holes (pore space) between the grains. This material is often referred to as spar. Some oolitic limestones, for example Ketton and Portland limestone, are held together by tiny pieces of spar, mainly at the points where adjacent ooliths touch. This gives the stone rigidity, and examinations will show the ooliths stacked tightly together. Other oolitic limestones, like Bath Stone, have their original pore space completely filled with spar, so that the ooliths at the cut surface of the stone may fall away to reveal a surface texture dominated by the concave moulds in the enclosing spar – like close-packed egg-cups on a tray. The first type is referred to as a grain-prominent oolitic limestone, and the second as a spar-prominent oolitic limestone.
The major Jurassic limestone formations that run from the south-west to the north-east of England were formerly referred to as the Lower, Middle, and Upper Oolites. The Lower was the most widespread and economically important, comprising the Great Oolite of the Bath region, and the Inferior (= subjacent) Oolite near Cheltenham. These formations contain many different varieties of limestone of which only some are oolitic, but the name was applied to the whole sequence. Architectural historians began to refer to this swathe of country as the Oolite Belt, and so, today it is commonplace to refer to all limestone as oolitic (when in fact, it isnt.)
Bath Stone and Portland Stone
Bath Stone and Portland Stone might be described as close geologic cousins. Bath Stone is a spar-prominent oolitic limestone. As is typical for the formation of limestones, in the Jurassic Period, the Somerset area was under shallow sea-water. Gradually, tiny grains of calcium carbonate rolling around on the bottom began to pick up layers of lime, grow in size and compact together in layers of marine sediment. Over the course of approximately the next 150 million years, pressure from consecutive layers of sediment caused those grains to form the limestone.
In much more recent history, two famous types of limestones have been quarried in the wider area of Somerset: Bath Stone and Portland Stone. Like Portland, Bath has given its name to a stage of geologic time - the Bathonian - which ran from 168.3 million to 166.1 million years ago, which was in the middle of the Jurassic era. Bath Stone can be distinguished by its golden-honey coloured rock, as opposed to Portland Stone, which is more grey in appearance. Bath Stone was originally extracted close to Bath but these days is mostly mined in the Corsham area. The majority of Bath Stone quarries are actually located underground, and have traditionally been referred to as underground quarries rather than mines. The reason seems to be simply that the people working in them wanted to distinguish themselves from miners of other minerals (such as tin, lead and coal), who they considered less skilled. During the 1840s, saws known as razzers and frigbobs were introduced that remained in use until well into the 20th Century.
The oolitic structure of Bath stone and its sparry calcite matrix are not strongly bound to one another and when the stone is fresh with a bit of moisture (or ‘quarry sap’) it is relatively soft. Because of this, it could be cut from the ground using hand picks and saws efficiently enough to make it a viable building material.
When the stone loses its quarry sap, cutting it becomes more difficult. It is generally thought that the stone surface hardens over time. It is not known exactly why; possibly, hydrated minerals migrate to outer surfaces as the stone dries and carbonate or alter to less soluble materials that provide added cohesion. Bath Stone is a hardy rock: it is generally accepted that the ooliths weather out of the matrix of Bath Stone so there is a persistent gradual surface denudation that is estimated to be around 3-4mm every hundred years under mild weathering conditions.
Both Bath and Portland Stone has a fairly uniform appearance, and both can be squared (cut) in any direction without compromising their strength. This makes both useful building stones for large and small projects.
A cross section of Bath Stone will show the relatively large egg-shaped ooliths in Bath Stone, with a visibly coarse texture to much of the stone. The matrix typically comprises coarsely crystalline calcite or ‘calcite spar’, which results in a closed, dense texture.
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
Questions:
- Look at the Bath Stone upon which the sundial is mounted. Describe the surface, colour and the texture of the stone.
- What can you say anything about its origin? What conclusions can you deduce from your observations?
- Why does the stone have pitted holes in its surface?
- What percentage of calcium carbonate would you say compromises a sedimentary limestone?
- Look at the rest of the building. Has Bath Stone been used elsewhere? What leads you to this conclusion?
- Look at the sundial. What time is it?
- Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache.
Good luck, and thanks for visiting this EarthCache!
