The village of Wickham probably developed from a small Anglo-Saxon farming community. A Saxon church was recorded here which was, in all likelihood, a wooden building on the same site as the present church. This medieval church which stands here probably dates from the 13th century, although stones with 12th century carvings and crosses have been found there. Originally this small and simple church was a 'chapel of ease' to St Nicholas', Plumstead, since Wickham (or East Wickham as it became known) was in the parish of Plumstead until 1852. The 'old' church is all that remains of the village. A new church was opened on the site in 1933 to accomodate the growing suburban population (see Church Micro 3686…Welling-St Michael). Since 1969 it has been used by as a Greek Orthodox Church and known as Church of Christ the Saviour.
This church is a Grade II listed building and English Heritage describes it thus:
Small single-cell church, of flint, rubble and brick, with gabled roof and prominent bellcote at west end. Probably of C12 origin, judging from lancet window in chancel; east window has 3 lights with cinquefoil heads within square frame; nave has cinquefoil-headed windows; west door within pointed brick arch. Tiled roof; shingled bellcote carrying weather vane added in 1897. Restored in 1926. The fittings have been largely removed.
It is unusual for brick to be used in the construction of churches of this age, but as a man-made entity, the brickwork is not of consequence for the purposes of this Earthcache. The rubble described in the construction of the church actually comprises of many different types of stone. This Earthcache will give you the opportunity to examine some of them and the weathering that has impacted on them in the past 800 years.
Conglomerates and breccias
Conglomerate Breccia
A conglomerate is a rock consisting of individual clasts (or fragments of pre-existing minerals and rock) within a finer-grained matrix (mass of material) that has become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments. These are distinguished from breccias, which contain angular clasts. Both conglomerates and breccias contain clasts larger than sand (>2 mm).
Conglomerates can be classified by the dominant clast size.
Granule conglomerate 2–4 mm
Pebble conglomerate 4–64 mm
Cobble conglomerate 64–256 mm
Boulder conglomerate >256 mm
Breccias are classified according to the rocks which form the clasts, or the way in which they were formed. Thus we talk of sedimentary breccia, tectonic breccia, igneous breccia, impact breccia and hydrothermal breccia.
Chalk
Chalk is composed of the skeletal remains of the bodies of minute marine animals, which can only accumulate uncontaminated by sediments in the deep ocean, away from land. The chalk is formed by the gradual accumulation of tiny calcite plates shed from micro-organisms called coccolithophores. In the right conditions these settle, which builds up on the sea floor. This is then transformed into rock by geological processes: as sediment builds up on top, and as the sea floor subsides, the lime mud is subjected to heat and pressure which removes the water and compacts it into rock.
It is these colourless skeletons which give chalk its white colour. Most chalks formed during the Cretaceous period, between 100 and 60 million years ago, and chalks of this age can be found around the world. The Cretaceous chalks record a period when global temperatures and sea levels were exceptionally high.
Chalk is a member of the limestone family of rocks, but while chalk contains a large amount of calcium carbonate, limestone contains more than 50% impurities. Limestone is thus usually much harder than chalk, and has been widely used for building. Chalk is much softer and has been little used for building. However, you will find chalk blocks in the walls of the church.
Flint
Flint is a hard, sedimentary form of the mineral quartz, categorized as a variety of chert. It is found mainly as nodules and masses in sedimentary rocks, such as chalks and limestone. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and usually has a glassy or waxy appearance. A thin layer on the outside of the nodules is usually different in colour, typically white and rough in texture.
It is not fully understood how flint is formed, but it is thought that it occurs as a result of chemical changes in compressed sedimentary rock formations, during the process of diagenesis. This describes the process of chemical, physical, or biological change in a rock’s original mineralogy and texture, which takes place at relatively low temperatures and pressures.
Flint has been used since antiquity up to the present day as a material for building stone walls. It was most common in parts of southern England, where no good building stone was available locally, and brick-making not widespread until the later Middle Ages. Knapping describes the shaping of flint, through the process of striking it to manufacture tools or to produce flat-faced stones for building walls. A hammer or pick is used to split the flint – often in half to create two stones with flat circular faces.
Weathering
Now we’ve considered some of the rocks used in the construction of the church, it’s time to consider the forces that have been acting against them in the past 800 years or so.
Mechanical weathering
This is the process of physical forces acting on the rocks to break them down. One type of mechanical weathering is freeze-thaw. This occurs when the water inside of rocks freezes and expands. That expansion cracks the rocks from the inside and eventually breaks them apart. The freeze-thaw cycle happens over and over again and the break finally happens. It is also known as frost wedging. Another important kind of mechanical weathering is salt wedging. As water enters the holes and cracks in the surface of rocks, it carries salt dissolved within it. The water evaporates, leaving the salt behind. Over time, these salt deposits build up, creating pressure that can cause rocks to split and weaken. Temperature changes also cause mechanical weathering. As temperatures heat up, the rocks expand and as the temperature cool, the rock contracts. The effect can be the weakening of the rock itself. Abrasion is the grinding action of other rock particles due to gravity or the motion of water, ice or air.
Chemical Weathering
Chemical weathering describes the effect of weathering on molecules and atoms. As with all chemistry, the greater the surface area of an object, the more chemical reactions can take place. For these chemical reactions to happen in nature, moisture, and heat must be present. Oxidation makes rocks softer. Many rocks contain iron and the process of oxidation is similar to an iron bar rusting. Hydrolysis is process affecting silicate and carbonate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. It causes rocks to expand, exacerbating mechanical weathering. Acidification occurs when rainwater, which is naturally slightly acidic because carbon dioxide from the air dissolves in it, falls on the rock causing a chemical reaction with the minerals present in the rock. Limestone and chalk are susceptible to acidification because they contain high levels of calcium carbonate.
Biological Weathering
Biological weathering encompasses the effect of animals and plants on the landscape. In its simplest form, this may be the effect of roots digging in and wedging rocks, or animals burrowing. But it also entails the molecular breakdown of minerals through the release of acidic compounds by plants. One of the most significant of these is lichen, a combination of fungi and algae. These secrete acids, which break down the surface of the rock. The tiny fissures thus formed accelerate other types of weathering.
Phew!
Got all that? Time to apply your knowledge.
QUESTION 1 (at given coordinates)
Go to the wall at the front of the church. Look at the stone used to construct the wall to the right of the door. What type of stone do you see? What, if any, effects of weathering can you observe?
QUESTION 2 (at Waypoint Q2)
Go to the north wall of the church, and look at the stone that forms the wall from the ground to a height of about 40cm at the coordinates for Waypoint Q2. What type of stone do you see? What, if any, effects of weathering can you observe?
QUESTION 3 (at Waypoint Q3)
Now move to the east wall of the church, and look at the stone immediately above the main window on the the top left corner (not the stone of the window surround, but the one forming the wall) at the given coordinates for Waypoint Q3. What type of stone do you see? What, if any, effects of weathering can you observe?
OPTIONAL ELEMENT
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Sources
http://en.wikipedia.org/wiki/Flint
http://en.wikipedia.org/wiki/Diagenesis
http://en.wikipedia.org/wiki/Knapping
http://en.wikipedia.org/wiki/Conglomerate_%28geology%29
http://en.wikipedia.org/wiki/Breccia
http://www.belchalwell.org.uk/chalk.asp
http://www.whitecliffscountryside.org.uk/index.php?id_sec=2&id_sub=2
http://www.discoveringfossils.co.uk/chalk_formation_fossils.htm
http://www.bbc.co.uk/bitesize/ks3/science/environment_earth_universe/rock_cycle/revision/6/
http://www.kidsgeo.com/geology-for-kids/0065-forces-of-weathering.php
http://en.wikipedia.org/wiki/Weathering