Chip off the Old Block
What you are looking for:
You’re looking for an old milestone, which once doubled up as a mounting block for horse riders on Flemingate in Beverley, East Yorkshire. Flemingate once formed part of the Beverley to Hull turnpike (toll road), one of the first turnpikes in East Yorkshire, built around 1750. It's likely that the milestone was placed in the early 1800s. The original cast iron plaque which read ‘Hull 8 Miles’ fell off and went missing some years ago, a clue to what geological processes are happening here.
Weathering
Weathering is the process of rocks and minerals chipping into smaller rocks, or breaking down into sediments or compounds, from parent rock structures. It differs from erosion as the process is carried out in-situ, i.e. the material weathered from a parent rock is not subsequently transported away from it by an agent such as water in a river, or ice in a glacier.
There are two broad terms for identifying the type of weathering which is taking place: Physical Weathering and Chemical Weathering.
Physical Weathering
Physical weathering is the mechanical breakdown of rocks without altering their chemical composition. This could be through biotic or abiotic agents. Physical biotic weathering occurs when living organisms, such as plants or animals, weaken rock structure. Examples include plant roots driving into cracks in a rock, or animals burrowing into rock material. Physical Abiotic weathering covers physical weathering processes not involving living things. Some examples include: thermal stress, the mechanical effects of heat weakening rock structure through expansion and contraction; freeze-thaw action, ice expanding in cracks or permable rocks forcing them to open wider and lose their structural integrity; and the physical impact of wind and rain on a rock’s surface.
Chemical Weathering
Chemical weathering occurs when substances react with the chemical makeup of rocks, changing their chemical structure. A common example is when alkaline rocks, such as limestone and chalk, react with acidic compounds naturally found in rainwater (e.g. dissolved carbon dioxide) causing them to dissolve into calcium and carbonate ions, that are no longer able to hold their structure.
Another important consideration when identifying weathering, is abrasion from human use. Constant impact can replicate natural physical weathering processes, leaving divots in steps or other well used surfaces.
Rock Types
Certain rock types are often more resistant to weathering than others due to how they formed. The table below identifies three broad rock categories, describes each type of rock's structural characteristics in relation to it's formation, and provides a brief overview of some identifying features.
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Rock Type
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Formation & Structural Characteristics
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Identification
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Igneous
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Molten magma cools and produces solid rock. The quicker the magma cools, the smaller the mineral crystals which form in the rock.
The rocks produced are often very dense and robust, thus are often used as flooring and work surfaces to resist constant wear.
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Crystalline
Tight interlocking grain structure
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Sedimentary
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Eroded sediments in alluvial systems (rivers) are deposited in areas of low stream power, such as when a river reaches the sea or a lake. The deposited material slowly builds up over time. Eventually, sediments deep underneath the building pile are exposed to intense pressure. The pressure causes sediments to ‘fuse’ into a rock, under compaction and the growth of various salt crystals.
The rocks produced are often grainy, and have a low resistance against being worked. This makes them a useful building material as they can be easily cut into blocks or shapes, but are not necessarily sturdy.
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Rough texture
Loose grainy structure
Layering
Often contain fossils (including plants and animals, but also riverbeds and ripple marks)
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Metamorphic
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Rocks forced deep into the earths crust by tectonic movements, exposes them to extremely high pressure and temperature. The heat and pressure are not enough to melt the rocks, but enough to change them chemically (similar to boiling an egg).
The rocks produced are dependant on the parent rock metamorphosised. Commonly, metamorphic rocks are robust and nonpermeable, making them a useful building material, such as roofing slate.
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Characteristics of metamorphic rocks range dependant on the parent material metamorphosised.
They are commonly smooth, crystalline and have a ‘squashed’ texture.
Some may retain sedimentary layers (slate)
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Logging Requirements
Being an EarthCache, you will need to answer some questions based on the information provided in this listing and from the physical structure at GZ to claim a find. Please use the geocaching message facility easily accessible from the geocaching app or via my profile to send your responses. Alternatively, you may contact me via email, also found under my profile page Once you have submitted your answers, please log a find, there’s no need to wait for my reply.
1.i. Using the cache listing, identify the type of rock you think the milestone is made from.
1.ii. Thinking about how long the milestone has been at GZ, how has the type of rock used to build it affected the speed which it has weathered?
2.i. The milestone has weathered in a particular way. What structure do you think has caused this pattern? (Hint: look up)
2.ii. How does your answer to 2.i. give clues to the type of weathering taking place? Include your analysis on what type of weathering you think is happening to the milestone.
3. Include a photo in your log taken at GZ of you, your username, or GPS.
I will follow up logs without answers or photographs with a message on the message centre. If I don't recive a reply within a reasonable timeframe, your log may be deleted. You can't get your answers wrong, no answers are 'wrong' as such. The idea is to provide an answer informed by the information given in this listing. I will try to respond to all messages as soon as possible.
I hope everyone enjoys my first EarthCache!