Welcome to Grandad's Clock and Chair!
This is an urban EarthCache in which geocachers are invited to examine a modern sandstone sculpture in Bradford city centre. The learning point of this EarthCache is to get the geocacher to become familiar with sandstone and draw conclusions about the origin of the rock here.
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:
clastic: rocks made from broken bits of other rocks
matrix: the geological material in which something is embedded
arkose: a type of sandstone containing at least 25% feldspar
mafic: a mineral or rock that is rich is magnesium or iron; often dark coloured
Grandad’s Clock and Chair is a sculpture in Bradford city centre that was installed in 1992. The artist, Timothy Shutter, has carved a mill owner's office with a comfortable chair, mirror and grandfather clock from sandstone on Chapel Street, Bradford, near the corner of Peckover Street. The piece was commissioned by Bradford Council and the Little Germany Action Group in 1991 after Shutter's design won in a sculpture competition. The piece cost £5,000. According to the artist, “the work looks back to the past, but the swinging pendulum of the clock indicates that time does not stand still and the past has an important contribution to make to the future.”
Since this is an urban EarthCache in the middle of Bradford city centre, there are no cliffs, quarries or topographical features in the vicinity that can reveal the significance of where the sandstone 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 sculpture 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 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.
All three types of rock make up the Earth’s lithosphere, the outermost layer. The lithosphere averages about 100 kilometres in thickness.
Sandstone
Sandstone (sometimes known as arenite) is a clastic sedimentary rock composed mainly of sand-sized mineral or rock grains. This 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 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.
Let’s consider the composition of sandstone. Sandstones are clastic from the origin they comes from. They are formed by cemented grains of rock fragments, (mono-minerallic crystals.) The additional minerals that binds the cementing together are usually calcite, clays and silica. The sand grains are usually in the 0.1 mm to 0.2 mm range, 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. and made of quartz. The types of sandstone whose grains are not visible to the naked eye are sometimes called siltstones and shales, also known as argillaceous sediments. The types of sandstone whose grains are visible are called breccias and conglomerates, and are termed rudaceous sediments. The degree to which the grains are rounded is a sign of how far they were transported, though this may not be apparent 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.)
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 ("wacky"). A well-sorted sandstone (little matrix) with little cement is called an arenite. Another way to look at it is that wacke is dirty and arenite is clean.
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.
The rock formations of sandstone often come from areas where water is 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. The layering of sandstone is one such clue as to the environment it was formed in. Fluid, irregularly-shaped patterns with wavy layers indicate deposits from sand dunes while more regular layering indicates 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.
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!
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 composition of the sandstone. How does the texture feel? If individually visible, how big are the grains? Therefore, is this sculpture made from argillaceous or rudaceous sediments?
- Is there a variety of clast size visible, or are they all mostly the same? Therefore, is the sandstone well or poorly sorted? Is it arenite, or wacke?
- What colour is the sandstone? What does this suggest about the minerals in it?
- Can you see any layering in the rock? Can you see any fossils? Therefore, do you think this sandstone formed in a marine or terrestrial environment ?
- 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?
- 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!
