The Church
This is on old Saxon church which was given by Queen Emma to the Bishop of Winchester in the early eleventh Century, though it was probably a wooden building then. Formerly known as Church of St Michael and All Angels the current structure dates back to the thirteenth century. It shows several phases of extension and displays excellent examples of three local building stones and one slightly more travelled building stone.
The Geology
The rocks of the Earth can be classified into three broad types depending on how they were formed. These are igneous, sedimentary and metamorphic. We will just consider sedimentary rocks here.
Sedimentary rocks form an extensive, but generally thin veneer across the Earth’s crust. Sedimentary rocks are formed by the deposition and subsequent consolidation and cementation of mineral material. How the mineral materials are deposited gives us three different types:
Clastic sedimentary rocks are made of fragments of other rocks, or grains of minerals and give us mustones, sandstones and conglomerates.
Biochemical sedimentary rocks are those composed of the remains of animals or plants and comprise most limestones, including chalk, coal and some deposits of chert.
Chemical sedimentary rocks form when minerals become very concentrated in water and are then precipitated. Deposits include rock salt, oolitic limestone (made of grains that have grown in warm shallow seas) and flowstones (like stalagmites, stalagmites).
The size of the grains and particles that make up Sedimentary rocks are also used to described them, this giving us:
Mustones, fine grained clay, silt and mud, with particles less than 0.0625mm in size.
Sandstones, made of sand sized grains (mineral is often quartz, but can be other minerals) between 2 and 0.0625mm.
Conglomerates, particles larger than 2mm, so pebbles, gravel and boulders of rock fragments.
Under the sea, and in lakes, the burial of sediment under more and more layers of further sediment squeezes, compresses and consolidates the material. Percolating water provides minerals like silica or calcite which are precipitated between the grains cementing them together. This can give extensive layers of uniform rock perhaps extending many 10s or even 100s of miles.
Other sedimentary rock can be more localized in extent. Conglomerates are less common in the geological record that other sedimentary rocks. Whilst they can be formed in many different environments, such as sea shores, rivers, deserts and mountains these as are generally more restricted in extent than other sediment deposition regimes.
This EarthCache looks at three of the four building stones used in the construction of the church. Starting with the oldest first, the four rocks are:
Oolitic limestone, with shell fragments in some blocks, likely to be from Bath or Portland and dating from the Jurassic approximately 150 million years ago.
Soft to touch almost smooth Chalk, from a local quarry and of the Cretaceous, roughly 80 million years ago.
Silica cemented sandstone, also called sarsen stone, from local fields, formed in the Paleogene about 50 million years ago.
Conglomerate, an unusual iron oxide cemented river gravel, from local fields, formed since the last ice age in the Neogene Period less than 0.5 million years ago.
The EarthCache
Take a look at the north side of the church at the listing coordinates, by the distinctive window flanked to the lower left and lower right by smaller infilled windows. Three of the rocks used to construct the church can be seen here.
Starting with the window surround, of the existing and two infilled ones:
Q1) Describe the appearance, texture and colour of the stone used.
Q2) What type of sedimentary rock do you think this is and why do you think it was used for this part of the church?
Immediately to the bottom right hand side of the window is an irregular block of stone:
Q3) Describe the appearance, texture and colour of this block.
Q4) What type of sedimentary rock do you think this is and why do you think it was used for this part of the church?
Immediately to the left of the window and forming part of the infill to the old window are three blocks of a third type of stone:
Q5) Describe the appearance, texture and colour of this block.
Q6) What type of sedimentary rock do you think this is and why do you think it was used for this part of the church?
Send your answers through my profile at geocaching.com either e-mail or message center. You don't have to wait for me to approve your answers. Once you send your answers feel free to log this EarthCache as found. If there are problems with your answers, I will contact you.
Sources:
Berkshire Geoconservation Group Local Geological Site Information Sheet www.berksgeoconservation.org.uk
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For full information on how you can expand the Church Micro series by sadexploration please read the Place your own Church Micro page before you contact him at churchmicro.co.uk
See also the Church Micro Statistics and Home pages for further information about the series.
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