This cache is part of a series of caches which are placed at or near village halls and community centres.
For details and statistics about the Village Hall Series please visit Village Hall Series website

For this EarthCache, geocachers are invited to examine the wall of a village hall in the town of March. There is a carving there, built into the wall that has been made out of sedimentary rock. The learning point of this EarthCache is to get the geocacher to become familiar and be able to distinguish between the three major types of sedimentary rock.
 
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.

The hall is located immediately adjacent to the spectacular St Wendreda’s Church, which is well worth a visit if you’re in March.

 

Keywords for this lesson:
 
solution - a liquid mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent).
precipitate - the process that separates solids from a solution
 

Welcome to St Wendreda’s Hall!

St Wendreda’s Hall serves as the community building attached to the adjacent church of St Wendreda in March, Cambridgeshire. There are some elements of the church hat date back to the 14th century, but much of the building dates from the early 16th century. The tower, spire and church are built of a pleasing mixture of stone, flint and brick.

The hall is a modern building, erected in 2006. However, like the church, it contains much older elements and it is at one of these that this EarthCache examines.

As you look towards the main entrance on the hall, to the right you will see an unusual carving set into the modern bricks. This apparent plaque is in fact an eighteenth century headstone depicting angels sorting souls on the day of judgement. 

The image shows souls responding to the call of the trumpet on the day of judgement, and being sorted according to their deeds and actions in life. The angel on the left holds the book in which all virtuous lives are recorded. All those not written in the book are thrown into Hell of fire by the angel on the right who holds the trumpet and a sword.

In the immediate vicinity of St Wendreda’s, there are no cliffs, quarries or topographical features in the immediate vicinity that can reveal the significance of where the stone 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 fountain will give some insight to it's provenance (the kind of environment in which it was formed.)
 
For this EarthCache, we will look at rocks in general, then focus on sedimentary rocks.

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. Limestone is an example of a sedimentary rock.
 
All three types of rock make up the Earth’s crust.

Sedimentary rocks

Sedimentary rocks are formed by the accumulation of sediments. 

There are three basic types of sedimentary rocks:

Clastic sedimentary rocks are often formed by mechanical weathering. Mechanical weathering is the process through which large rocks are broken into increasingly smaller pieces. Also known as physical weathering, the process normally happens near the Earth’s surface. Mechanical weathering doesn't change the chemical nature of the rocks. Breccia, conglomerate, sandstone and shale are examples of clastic sedimentary rocks.

Chemical sedimentary rocks are often formed when dissolved material precipitate from solution. As minerals break down, compounds separate as they dissolve in water. Water that precipitates from the atmosphere has very little material dissolved in it, but water can become highly enriched in solute as it flows across the surface or into the Earth’s crust. Changes in physical and chemical conditions (for example, temperature, pressure, change in carbon dioxide, or biological activity) can cause minerals to precipitate. In desert environments, water can evaporate leaving salts to precipitate. Underground, water enriched in solute may gradually precipitate to form speleothems (cave formations.) The process of cementation hardens sediments into sedimentary rocks. Rock salt, iron ore, chert, flint and some limestones are examples of chemical sedimentary rocks.

Organic sedimentary rocks form from the accumulation and lithification of plant or animal material, such as leaves, roots, bone, and coral. This material builds up over time, and over the course of millions of years, turns into rock. Limestone might technically be classified as an organic sedimentary rock but is usually grouped as a chemical sedimentary rock. Coal and some limestones are examples of organic sedimentary rocks. 

(More on limestone in a minute...)

Here are three commonly found examples of sedimentary rock. Each example is from one of the types of sedimentary rock. The fountain is made of one of the following three rocks:

coal

coal – this variety of sedimentary rock formed in ancient swamp environemnts, full of organic matter that formed swampy sediments or peat beds which was then lithified. It is black, soft, and rich in fossils. Coal is combustible. The most significant uses of coal are in electricity generation, steel production, cement manufacturing and as a fuel.

sandstone
 

sandstone - 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.

limestone

limestone – this rock often formed when sediment gathered on the floor of ancient warm, shallow seabeds. It principally consists of clean-washed sand-sized grains of calcium carbonate. Some of these grains came from the shells and calcified skeletons of sea creatures. Others grew there, on the seabed, as the mineral calcite was precipitated around smaller particles (in the same way that limescale might build up on the inside of a kettle.) (This is why some limestones can be considered chemical and others biological sedimentary rocks.) 

Please note: Limestone and sandstone are sometimes mistaken for each other; sandstone has a visible stratification into layers whilst limestone does not.. Limestone does not have the stratification pattern that sandstone does. If you can see horizontal lines through the stone indicating these layers, it is likely sandstone and not limestone. Also, since sandstone is made up of many types of rock and sand, it can be many different colours. Freshly-quarried limestone is often closer to white due to its chemical makeup. 

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.
 
1. Look at the angel carving. It is made from a sedimentary rock. Describe the surface, colour, grain size and texture of the stone.
2. If any fossils are present, describe them.
3. What rock was used to carve the carving?
4. What type of sedimentary rock was used to make the carving? (clastic, chemical, organic?) Please explain your answer.
5. Look at the other carving set into the wall on the other side of the door. This other carving is made of the same sort of rock as the angel carving, but was quarried several centuries later. What differences do you notice when comparing it with the angel carving?
6. 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!

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If you would like expand the series and place a cache of your own
please request a number using the contact form on the website or email
GeoJaxx at villagehallseries@gmail.com
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See the bookmark list for all the caches in the series.
Facebook group: Village Hall Series – Geocaching

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