This is an urban EarthCache in which geocachers are invited to examine a statue in central Bath. The learning point of this EarthCache is to get the geocacher to become familiar with sandstone, in particular the local Pennant 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.
Welcome to the Rebecca Fountain!
Water is best!
The Rebecca Fountain, on the north side of Bath Abbey, is an Grade II historic drinking fountain dating from 1861. The fountain depicts a life-size female figure pouring water from an urn into a bowl with the inscriptions "water is best.” It was erected by the Bath Temperance Association in 1861, and the thought was to give people the option of free drinking water rather than going to a public house for refreshment. When first positioned it faced quite a few public houses in the High Street in a stand-off against the ‘Devil’s brew’.
The statue is made of white Sicilian marble and was built in 1859. According to the inscription, it was not installed until 1861. Local fountains like this became common across Victorian Britain, set up by the various temperance societies, but ‘Rebecca’ was one of the first. She was installed around the same time as a similar public drinking fountain located on Walcot Street. Rebecca’s classical style was unusual for its day, and clearly a nod to the nearby Roman baths. For its part, the inscription on the fountain is also found (in Greek) on the front of the Pump Rooms.
"Water is Best" in Ancient Greek - at the Roman baths
The story of Rebecca appears in Genesis 24:15-20. Rebecca really did provide clean drinking water to the people of Bath until the 1980s, and was also a favourite place to meet friends… or wait for the bus.
For the purposes of this EarthCache, rather than Rebecca herself, we shall be examining the plinth and steps of the fountain. These are made from a type of sandstone called Pennant Stone.
First, let’s look at rocks themselves, and then in particular sandstone.
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.
Sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral or rock grains. ‘Clastic’ means it has come from pre-existing rocks, rather than from an organic or chemical origin.
A very simplified way of thinking of sandstone is being sand that has turned into rock. It forms when grains of sand from existing rock or crystals become cemented together over time. The formation of sandstone happens over two stages. First, sand (often, but not always, suspended in water) settles and accumulates. Second, this deposited accumulated sand is compacted by the weight of further deposits. You can identify and comment upon the rock by its distinguishing characteristics:
Composition
Since sandstones are clastic sedimentary rocks, they are formed by cemented grains of rock fragments. The minerals that form the cement are usually calcite, clays and silica. The sand grains are usually between 0.1-0.2 mm diameter, but can be as small as 0.625mm or as big as 2mm in size. Therefore, many of the individual grains may not be visible to the naked eye. Geologists call these argillaceous sediments. Conversely, sandstones whose grains are visible are termed rudaceous sediments. The degree to which the grains are rounded indicates how far they were transported, though the grains may not always be visible without a microscope. Sandstone feels gritty to the touch, like sandpaper. Clast size may be grouped as fine (0.06-0.2mm), medium (0.2-0.6mm) and coarse (0.6-2mm.)
Sorting
The variety of clasts within a sandstone is also a distinguishing feature. A sandstone comprising a mixture of clast sizes, and a lot of matrix is poorly sorted, while one comprising mostly clasts of the same size is well sorted. Many of the sand grains wont be visible to the naked eye, but if there is a wide discrepancy of grain sizes, this should be more evident. If matrix amounts to more than 10 percent of the rock, it is called a wacke (pronounced “wacky"). A well-sorted sandstone (with little matrix) with little cement is called an arenite.
Colour
Sandstone can appear in many colours, including tan, brown, yellow, red, grey, pink, white, and black. Most sandstone is composed of the minerals quartz or feldspar. These minerals are amongst the most common found in the Earth's lithosphere. The most usual colours for a stone made of sand are tan or yellow. These colours come from the mixture of the clear quartz and the dark amber-coloured feldspar. Iron oxide is a common impurity which can change the colour of the sandstone and change it from pink to dark red.
Origin
Sandstone often formed in ancient areas where water was present. Nevertheless, sandstone can form in marine or terrestrial environments. In this context, ‘marine’ includes beaches, the sea bed and deltas. ‘Terrestrial’ includes deserts and sand-dunes as well as aquatic, non-oceanic sources such as lakes and rivers. For example, the famous red rocks of the Grand Canyon in America formed in a desert setting. The location of the deposit determines the specific composition and grain size. One of the clues to the origin of sandstone is its layering. Fluid, irregularly-shaped patterns with wavy layers indicate they might be deposits from sand dunes while more regular layering might be more indicative of deposits from water. Another clue is fossils - these tend to be found in marine sandstones, though the energetic environments where sandstones form don't always favour preservation.
Cement
Another consideration of sandstone is its cement. Typically consisting of calcium carbonate or silica, the cementing agents are the materials that hold the sandstone together. The composition of the stone and the cementing agent used will determine the strength, durability and weather-resistant properties of the sandstone. The strongest and most durable type of cement is called silica (quartz) cement. This type of sandstone normally forms in environments with high energy currents, such as beaches, marine bars and desert dunes. The most common type of cement is calcite cement. This typically forms in patches and does not fill all the gaps within the stone. This makes calcite cement sandstone very porous. Calcite is also soluble in water, which can erode away the cement making the stone even more porous. Iron oxide is another common cement, and as previously discussed, gives the sandstone a distinctive red colour. Less common cementing agents include pyrite, barite and gypsum. These cementing agents form crystals between the particles of the stone. These cements produce a much softer type of sandstone with the particles able to be rubbed off the stone with your hand. In areas with cementing agents of a soft nature, such as clay and gypsum, the sandstone produced tends to also be soft in nature. This kind of sandstone isn't much use for building… or sculpture!
Pennant Stone
And so, back to Rebecca. She is rendered in white Sicilain marble, but the point and steps are carved from a type of sandstone called Pennant Stone. Pennant Stone is a hard to greenish sandstone. Pennant Stone is located widely in the South Wales coalfield, from near Llanelly in the west to Pontypool in the east. It is is also present in coalfields in the Forest of Dean and Bristol areas, and in the subsurface around the coalfields of Oxfordshire and Berkshire.
Pennant Stone is commonly used in the South Wales and Somerset areas as a roofing and paving material. Pennant Stone is very durable and resistant to erosion. Ringed staining due to the presence of iron (Liesegang staining) is often seen.
Pennant Stone formed in the Carboniferous Age, that began 358.9 million years ago and finished 298.9 million years ago. The formation where Pennant Stone is found also contains numerous seams of coal. Because of this, Pennant Stone can be distinguished quickly by experts from other types of sandstone by the small fragments of coal that is found in it. These fragments are very small, barely discernible to the naked eye, at under 1mm diameter.
In the formation where Pennant Stone is found, the base level is the Pennant sandstone bed. It’s quite thick, at over three metres wide. Higher in the formation, coal is found, and above that grey mudstone. Underneath the formation are grey mudstones and siltstones. Pennant Stone also forms the plateau surface of the coalfield, and the surface level of the coalfield valleys which over millennia have been deeply incised by water and ice. The highest level that Pennant Stone is found is at the flat summit of Craig y Llyn, (600 metres above sea level.) But here, Rebecca draws water from the well at around 25 metres above sea level.
Keywords from this lesson:
temperance - abstinence from alcohol
sedimentary rocks - rocks formed on the Earth’s surface from the products of weathering which then becomes cemented or deposited.
clastic sedimentary rocks - rocks formed from pre-existing rocks
argillaceous sediments - grains not visible to the naked eye
rudaceous sediments - grains visible to the naked eye
poorly sorted - a clastic sedimentary rock made up of a mixture of clast sizes and a lot of matrix
well sorted - a clastic sedimentary rock made up of clasts that are mostly the same size
wacke - a clastic sedimentary rock where the matrix is more than 10% of the rock
arenite - a well-sorted sandstone with little matrix
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.
Remember, you’re looking at the plinth and steps, not at Rebecca herself!
Questions:
1. Look at the composition of the Pennant Stone. How does the texture feel? If individually visible, how big are the grains? Therefore, is this rock made from argillaceous or rudaceous sediments?
2. Is there a variety of clast size visible, or are they all mostly the same? Therefore, is the Pennant stone well or poorly sorted? Is it arenite, or wacke?
3. What colour is the Pennant stone? What does this suggest about the minerals in it?
4. Think about the cement of this sandstone. Is the rock hard or soft? By observing the rock and from what you have learnt in the lesson, what cementing agent do you think is present? What makes you think that?
5. Please describe the identifying charecteristics of Pennant Stone, ringed staining or coal fragments, if you can see them.
6. Rebecca says, "Water is best." What is the other inscription on the fountain?
7. 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!
