The City Conduit EarthCache
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A piece of Chelmsford's history.
Before logging this cache, please send us the answers to the questions below
History of the Conduit
Chelmsford’s water supply originally came from a number of springs around the town (as it was then), and in 1683 was piped from these, to the High Street through a series of open channels and wooden pipes. In 1708 a public conduit was established in Conduit Square (now Tindal Square), to supply water to the general public. In 1814 this was replaced by a new, larger conduit, in the form of a Rotunda, to serve a growing population.
With the advent of other sources of water, and the beginning of piped mains supplies, the spring fed water supplies were no longer needed. The conduit was therefore moved to the junction of Springfield Road and the High Street, where it was fitted with a gas lamp on top, and served as a traffic island.
In 1940, due to the volume of motor traffic in the area, it was deemed to have become a traffic hazard. The conduit was therefore dismantled, moved to Tower Gardens – so named because of the (then) adjacent water tower in Admirals Park – and reassembled on its present site.
Construction
The conduit - which is a Grade II listed building, and thus susceptible to damage - is constructed from Portland Stone, and consists of a dome, supported by six tapered Doric columns on pedestals, surmounted by a circular entablature with triglyphs.
Note. (An entablature with triglyphs, is a frieze with a pattern of three vertical bars with six triangular pegs beneath, spaced at equal intervals. This pattern is only found in the Doric style).
Geological Rock Types
Sedimentary Rock
Sedimentary rock - such as Limestone, Portland Stone, is formed by the deposition over many years, of layers of sediment on the bottom of lakes and seas. This sediment is then compressed over long periods of time, and is consolidated into layers of solid rock - called Strata. These strata can often be seen as bands of rock in exposed cliff faces.
Portland stone is used in buildings such as the new London Stock Exchange, the National Maritime Museum in Greenwich, and the Ashmolean Museum in Oxford.
Sandstone, another sedimentary rock, has historically been used for many of the buildings in Edinburgh, (as it was the locally quarried stone), as well as buildings in other parts of the UK. It is found in a variety of colours, from buff through to pinkish-red, depending upon its composition.
Igneous Rock
Igneous rock - such as Granite, is formed by the solidification of molten rock (Magma), from within the Earth's crust. Different cooling environments lead to different crystal sizes within the rock. Rock which has cooled slowly beneath the Earth's surface has a large crystal size, whereas rock which has cooled rapidly on the Earth's surface - perhaps as the result of a volcanic eruption - has a small crystal size.
Granite, which makes up 70–80% of Earth’s crust , is formed of interlocking crystals of quartz , feldspar, mica, and smaller amounts of other minerals. It forms deep underground, thus cooling is slow, and larger crystals result in a coarse or mottled crystalline structure, easily visible to the naked eye. It has a colour range from light grey to pinkish red, depending upon the amount of each of the major constituents present. It is frequently used for the lower parts of building frontages, and memorials, as it is very weather resistant, and can be polished to give a smooth shiny surface. It is however, sometimes left relatively rough, for areas such as large buildings, and for steps. The best known granite structure in London is probably Tower Bridge. Metamorphic Rock
Metamorphic rocks - such as Marble, are formed by the effects of temperature and pressure on already formed rocks, over a long period of time, and generally deep within the Earth's crust. The severe conditions involved, result in the original mineral constituents of the rocks becoming unstable, and these then undergo metamorphosis and recrystallise into a more stable form.
Marble is formed from the action of heat and pressure on Limestone. This is not sufficient to melt the limestone, but enough to cause it to undergo crystal rearrangement. Marble is harder than limestone, and generally more weather resistant, it can be cut and shaped, and also be polished to a smooth shiny finish. Many colour variations are found, depending upon impurity types and levels, colours can range from almost pure white, to almost black. Perhaps the best known marble building in the world is the Taj Mahal in India
Portland Stone Geology
Portland Stone is a type of limestone, a sedimentary rock from the Jurassic period of the Phanerozoic Eon.
Geological Timeline
It has historically been quarried from extensive beds on the Isle of Portland in Dorset, and is a material that is still widely used in the construction of all, or parts, of many important buildings. Large reserves of stone are available on the Isle of Portland, and are still being quarried to keep pace with the current high demand for the material.
Section across the Isle of Portland
Portland Stone was formed from the shells of deceased marine mammals etc. - which contained significant amounts of Calcium - in shallow seas, close to land masses, at a time when, because of continental drift, Southern England was on a similar latitude to modern day Israel, and the water temperature was thus significantly higher.
Calcium carbonate (CaCO3), has a higher solubility in water containing dissolved carbon dioxide (CO2),i.e. having a lower pH. However when seawater is warmed, its capacity to hold dissolved gas is reduced, and carbon dioxide (CO2) is released into the atmosphere as a gas. This affects the chemical balance of the water, raising the pH, and the calcium carbonate forms as a precipitate, as shown in the following equation:-
Ca2+ aq + 2HCO3 aq <=> CaCO3 solid +H2O aq + CO2 gas
(A similar mechanism to the lime scale build up in a kettle, in hard water areas).
Many minute crystals of this precipitated calcium carbonate (mainly in the form of Calcite, plus some less stable Aragonite), accumulated to form a lime "mud" (called Micrite), which covered much of the sea floor. Particles of sand, or other materials such as further shell fragments, then acted as nuclei, these slowly became coated with layers of Calcite as they were moved around in the Micrite by wave action. The Calcite very gradually increased by accretion in concentric layers around these nuclei, over a long period of time, forming small calcareous spheres, generally of less than 1 mm diameter. (This is similar to, but much slower than, the way in which a snowball grows in size, as it is rolled around in the snow). Over time, countless numbers of these tiny balls, known as Ooids or Ooliths, became buried, and partially cemented together by more Calcite, to form the Oolitic Limestone we call Portland stone.
Oolitic Limestone
The amount of cementation in Portland stone, is such that the stone is sufficiently well cemented to allow it to resist significant amounts of weathering, but not to such a degree, that it can't be readily worked by masons using normal tools. These are two of the reasons why Portland stone is particularly favoured as a monumental, and architectural stone.
Portland Stone is well known for containing fossils, most - such as Portland Screws - are found in the upper, or Roach layer, fewers are found in the middle, or Whitbed layer, and the smallest number, and probably the least well defined, are found in the lower, or Basebed layer (these being the types of Portland Stone most commonly quarried and used).
Present Day
Today, Earth still has many limestone-forming environments.
Most are in warm shallow water areas, between latitude 30 degrees North, and latitude 30 degrees South.
Limestone - mainly from Aragonite - as opposed to the Calcite of Portland Stone - is forming in the Caribbean Sea, Indian Ocean, Persian Gulf, Gulf of Mexico, around some Pacific Ocean islands, and within the area of the Indonesian islands. There in the warm waters, abundant corals, shellfish, and other marine organisms, produce large amounts of calcium carbonate containing debris, that blankets the seabed, and is gradually producing extensive limestone deposits.
Before logging this cache, please send us the answers to the questions below
Please DO NOT climb on the conduit, it is a listed structure
Q1. What type of rock is Portland Stone - Igneous, Sedimentary, or Metamorphic.
Q2. From the number of fossils present, which layer would you say the stone of the Conduit came from.
Q3. (a).There is another stone used to surround the base of the Conduit. What do think this stone is, and how many sections of it are there.
(b). Is it the same type of rock as Portland Stone, if not, what type of rock do you think it is.
Q4. How many triglyphs are there on the Conduit.
You may log your find after sending the answers.
Acknowledgements
Dr. Ian West for permission to use his images.
Chelmsford City Council for permission to place this Earthcache
Additional Hints
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