UK Mega 2021: Lincolnshire
The Constable's Territory
These are pairs of mid 18th century boundary marker stones denoting the division between the territory administered by the City Council and that ruled by the castle constable. There was a strict curfew imposed upon citizens of Lincoln, who were not allowed inside the constable's territory after dark.There are three surviving pairs of these boundary markers, marking the limits of the Constable's district. As well as the pair here on Steep Hill, there is another pair on Eastgate and a third just to the north of Newport Arch. Each of these pairs is identfied by a waypoint within this EarthCache listing.
These Grade II listed boundary stones are tapered square pillars about a metre in height, with the City of Lincoln arms on the front face. They are 'ashlar' stones, meaning they are dressed, or finished, on all surfaces, having been worked with tools, rather than left natural or in rough, hewn blocks. They are often used in regular units laid in courses in place of bricks. These examples, however, are lone ashlar blocks. They are each made of a sedimentary stone (see below).
Sedimentary Rocks
Sedimentary rocks are one of three main types of rock, along with igneous and metamorphic. They are formed on or near the Earth's surface by the compression of ocean-based or land-based sediments or various processes. The underlying bedrock in the Lincoln area is a type of sedimentary rock known as limestone. There are several ways to distinguish between sandstone and limestone.
Texture
Limestone is made of calcium carbonate. Although it may contain some granular material, it is often set in a calcite matrix. By comparison, sandstone is always formed of grains.Close exampination with a hand lens or loupe will show it to be made up of tightly packed grains.
Colour
Limestone can be a variety of coloures, often grey, although it can also be white, yellow or brown. Sandstone can be found in a wide range of colours, depending on chemical content and the environment in which is was first formed.
Fossil Content
Limestone is often extremely fossiliferous, containing the trace remains of numerous marine life forms. These can include crinoids, and belemnites as well as the shelly remains of a myriad of intertebrate life forms as well as plants. While sandstone can also contain fossils, these tend to be of either land or oceon-based organisms, depending on where the sandstone was formed. For example, most dinosaur remains are found in sandstone. However, most sandstones do not contain fossils.
Weathering.
Another way to distinguish between limestone and sandstone is through the use of acid. Limestone tends to react strongly with acid as the calcite corrodes. Dropping a pebble of limestone into vinegar overnight, for example will result in bubbles of carbon dioxide appearing as the rock slowly corrodes. This is why limestone is very susceptible to damage by weathering from acid rain. Sandstone, which contains large amounts of quartz is far less reactive to acid.
Erosion
However, sandstone and limestone's strength often depends upon how tightly packed and cemented the grains are. For this reason, exposed surfaces of sandstone can be extremely vulnerable to erosion by wind and rain. Erosion is a physical process whereby natural forces like water, wind, ice, and gravity transport rocks and soil or particles thereof. Wind and rain, for example, can cause small ieces of rock or rock particles to flake away from the surface.
Your tasks
To complete and claim this EarthCache, you will need to complete these tasks, answer the questions and send your reponses to me using either the messenger or by email. You can log the EarthCache as soon as you have done this. I try to respond to all messages but this might take a few days depemding on what else I'm up to! However, logs received without a reasonable attempt having been made may be deleted after a while.
1. (a) Visit each of the pairs of boundary markers in turn and make notes on the following: texture, colour, composition, size of grains and of any fossils present.
(b) Look at the structure of these ashlar blocks. Look for evidence of a) weathering and b) erosion. Describe this evidence for me.
2. Using these observations, state whether these stones are made from sandstone or limestone.
3. From the least weathered/eroded to the most weathered/eroded, rank each of the pairs of blocks, A, B & C (see waypoints)
4. (Optional task) Post a photo of you (or your GPSr) showing a nearby landmark or view, without showing the boundary marker(s).
This EarthCache was placed as part of the Let's Linc Link community cache placement programme which is part of the celebration of the UK Mega 2021 here in Lincolnshire.
