Introduction
A metamorphic rock used to be some other type of rock, but it was changed inside the Earth to become a new type of rock in a process called metamorphism, derived from the greek words meta meaning change or trans and morph meaning form.
So a metamorphic rock is a rock which has been transformed from one type of rock into a different type of rock.
This EarthCache takes you to a location where you can see an interesting example of a metamorphic rock up close and then asks you, based on what you've observed and learned, to correctly identify how many million years ago it was formed as well as what it was before it became slate.
Logging Tasks
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Based on your observations at the cache location - specifically at the south-facing end of the building, which faces a red brick building - and the information on the cache page, please tell me:
- Which specific type of slate do you see at the cache location and how do you know this?
- What is the provenance of this slate i.e. what sort of sediments made up the protolith and when were they laid down?
- The protolith was subjected to low grade metamorphism - what sort of temperature range was involved in that process?
- What colour are the thumbnail-sized crystals embedded in the slate?
- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
Metamorphic rocks are formed through the transformation of pre-existing rocks in a process known as metamorphism (change in form).
The original rock, or protolith*, is subjected to heat and/or pressure which cause physical, chemical and mineralogical changes to the rock. Protoliths may be igneous, sedimentary or pre-existing metamorphic rocks.
*Protolith - taken from the greek words proto (first) and lithos (rock) is a word geologists use to describe rock before metamorphism i.e the protoliths of slate are mudstone or shale.
Slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived through low-grade regional metamorphism from an original shale-type sedimentary rock protolith composed of clay or volcanic ash.
In other words...
Millions of years ago, tiny sediments of mud, clay or volcanic ash accumulated on the sea bed and were compacted and stuck together by mineral cements to become a sedimentary rock called shale or mudstone. That shale/mudstone was subsequently subjected to heat and/or pressure and transformed into the metamorphic rock we see today - slate.
The heat/pressure acting on the rock caused minerals in the rock to line up parallel to each other, in a single direction, resulting in a pattern of repeating parallel bands in the slate known as foliation - hence the rock is described as being foliated. The slate is also fairly uniform in composition throughout - what geologists refer to as homogeneous.
Metamorphism
Metamorphism relies on heat or pressure - or both together and these things in turn rely on the depth of burial of the protolith.
I think a pictorial representation in the form of a diagram might help at this point and I particularly like the one below for the fact that it includes depth and pressure - but it is only a rough guide though as it's a simplified graphical representation of a more complex subject matter, what's sometimes referred to in scientific circles as a cartoon diagram or simply a cartoon 
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At the Earth's surface, physical and chemical changes that occur during the conversion of loose sediments into solid sedimentary rock are called diagenesis. Roughly speaking, diagenesis occurs at temperatures below 200°c.
Temperatures capable of supporting metamorphic processes range between 100°c and around 900°c - so there's some ovelap with diagenesis at the lower end of that range rather than a hard boundary.
As indicated by the cartoon, the temperatures/pressures which give rise to low grade metamorphism tend to be at the lower end of the spectrum compared to those which give rise to high grade metamorphism.
The melting point of rocks is around 700°c to 900°c, depending on the type of rock and the amount of pressure they are subjected to - so no hard boundary there either. Rocks which do melt though are outside the conditions for metamorphism because a fundamental aspect of metamorphism is that rocks stay solid throughout the process.
Which Slate Is This Slate?
The source rock of a metamorphic rock is known as the protolith.
The source of sediment from which a sedimentary rock is formed is known as the sedimentary rock's provenance.
We know that the protolith of slate is a shale-type rock, but what is the provenance of that shale-type rock? The answer to that question is - it depends! .
The slate at the cache location was mined or quarried in the county of Cumbria in the North of England. Cumbrian slate comes in two distinct types - with two distinct geological histories.
The provenance of the blue-grey / black type of Cumbrian slate is mud and silt, laid down during the Silurian Period - some time between 443 and 417 million years ago.
The provenance of the green type of Cumbrian slate is volcanic ash, laid down during the Ordovician Period - some time between 495 and 443 million years ago.
The processes of diagenesis turned the mud into a sedimentary rock known as mudstone and the volcanic ash into a rock called tuff.
The provenance of the green type of Cumbrian slate is therefore considerably older than that of the blue-grey type. Interestingly though, their sedimentary protoliths both metamorphosed into slate around the same time, at the end of the great mountain-building period known as the Caledonian Orogeny around 400 million years ago.
One of the commonly used names for the blue-grey / black type slate is Burlington slate and for the green variety it's Westmorland Green slate.
Given that identifying the type of slate at the cache location is a Logging Task, I'm obviously not going to tell you which one it is here - but the above information should enable you to correctly identify it for yourself 
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I Am A Passenger...
The slate is a metamorphic rock but it's carrying a passenger or a hitchhiker if you like, in the form of some thumbnail-sized crystals incorporated into the body of the slate.
These crystals are very easy to spot as their colouration is distinctly different from the surrounding slate.
If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there .
Please submit your logging task responses before posting your log.