Church Micro 2004…West Hoathly

West Hoathly church has been here for 900 years, but has been modified many times over the years. The sandstone for the church is local in origin and the church is mainly made of rubble - rough lumps of stone - not carved square blocks (which are called ashlar).

EarthCache Lesson and Questions

This EarthCache will examine sandstone, a stone used regularly for building. The EarthCache lesson will include features of the stone that can have an effect on its appearance and use, including the effects of mineral contamination, the changes in the core material and properties which make it susceptible to weathering and erosion.

In order to log this EarthCache please send me the answers to the following questions, through the Message Centre or by e-mail.

1. Although the church is mainly built of one material, sandstone, it has many different colours. Standing at the door at the west end of the church list three different coloured sandstones that you can see and suggest what might cause the variation in colour.


2. Move round to the porch on the south side and examine the blocks at about 1.5m up on the left of the porch. These are much newer blocks than the rest of the church and have been tooled to give a heavily patterned surface. What faint feature can you see on the 3rd and 4th rows of new blocks up on the left pillar. Can you suggest what may have caused this?


3. To the right of the porch, behind a large tomb, there are signs of erosion in the lower rows of stones. Which colour of the sandstone appears to be the most damaged? What do you think may have caused the damage?

Please add a photograph to your log taken at the church showing something associated with you and a background that doesn’t give away any answers. Your face doesn't need to be in the photo.

Formation of Sandstone

Sandstone is a sedimentary rock, that is one where the rock is laid down as a result of small particles settling into layers and then being compressed and glued together. Sandstone comprises about 20–25% of all sedimentary rocks. It is composed mainly of sand-sized (0.0625 to 2 mm) silicate grains, cemented together by another mineral.

The silicate sand grains from which sandstones form are the product of physical and chemical weathering of bedrock. Eroded sand is transported by rivers or by the wind from its source areas to places where the topology of the land provides space for the sediments to accumulate.

As sediments continue to accumulate, older sand is buried by younger sediments, and it undergoes compaction and lithification. Lithification is the bonding of the individual grains to form a rock. There are several stages within this process, which occur at different depths.

Mechanical compaction mainly takes place at depths of less than 1,000m as the sand comes under increasing pressure from overlying sediments. Sediment grains move into more compact arrangements and the pore space (the gap between the grains) is reduced. Chemical compression, which can take place at depths of up to 2,000 metres, causes parts of the grains to dissolve where they are in contact with other grains allowing the grains to come into closer contact – effectively knocking off the rough edges from the grains. Lithification follows closely on compaction, as increased temperatures at depths of 2,000-5,000m hasten deposition of the cement that binds the grains together.

Colours of Sandstone

Most sandstone is composed of quartz or feldspar. As it is composed of light-coloured minerals, basic sandstone is typically light tan in colour. However, iron oxides, manganese oxides and other impurities can cause bright and contrasting colours in sandstones, which gives sandstone its unique character and ornamental desirability. The colours range from bright whites, reds, yellows, oranges and even purples and greens. These colours can change over time once the stone has been extracted for example as the minerals oxidise. It is unlikely that the front face of the church had the variety of colours that it is now showing when it was first built.

The colour of a sandstone also depends on the sand grains and the bonding material. An abundance of potassium feldspar often gives a pink colour. Fine-grained, dark-coloured rock fragments, such as pieces of slate, chert, or andesite, however, give a salt-and-pepper appearance to a sandstone. Iron oxide cement imparts tones of yellow, orange, brown, or red, whereas calcite cement imparts a grey colour. A sandstone consisting almost wholly of quartz grains cemented by quartz may be glassy and white.

Banding

Sandstone banding is due to layers of sand with differing characteristics that are deposited over time. Sandstone is formed in many deposits that are episodic in nature and the resulting layers can be very different from previous layers. Sometimes the sand is coarser or finer than the previous layer and this difference causes the banding or strata showing in the final rock. A place where this can be clearly seen is in Antelope Canyon in Arizona USA, pictured below. A much more subtle version can be seen at the church.

Weathering

Weathering is the deterioration of rocks over time by natural forces, causing small, or sometimes, large pieces to be detached from the main stone. Erosion is the removal of the broken pieces of rock from the stone by gravity, wind, water or air. Sandstone can suffer from weathering and erosion, although some of the sandstone in the church has been in place since around 1100, so it can be a slow process.

Weathering can take place by two main mechanisms:

• Mechanical – in which there is no chemical change in the rock, but the rock is acted on by the wind, water, ice and even plants.


• Chemical – in which the chemical make-up of the rock is altered, through contact with water, with or without dissolved chemicals, the air and plants.

Biological weathering can take place as either mechanical, in the case of a plant growing in a crack and enlarging that crack with its roots, or chemical, when soil organisms or lichens cause changes to local conditions.

Sandstone appears to be more susceptible to a form of weathering called contour scaling than other rocks. Contour scaling occurs when the rock fractures in a direction parallel with the front surface of the stone, and a thin layer of rock then breaks off. There are a few mechanisms which may cause contour scaling, but they all seem to be associated with water ingress. Sandstone is porous and thus when it gets wet the water can be drawn into the surface of the rock by capillary action for a small distance. Over long periods of time this water ingress and subsequent drying out may change the structure of the rock perhaps by depleting the cement between the sand grains or blocking the pores with precipitated salts or even due to freeze thaw cycles.