Merthyr Tydfil railway station is the terminus of the Merthyr Line, which connects the town to Cardiff. Merthyr Tydfil has a rich industrial heritage, historically known for its role in the iron and coal industries. Slate, widely used in construction and roofing across the region, is also an important part of the area's geological and industrial history.
Just outside the station, a slate wall provides an opportunity to observe the geological features of Welsh slate up close. This EarthCache will guide you in exploring impurities within the slate, such as calcite and quartz, which reveal clues about its formation, structure, and properties.
Slate is a metamorphic rock, meaning it has undergone a transformation due to heat, pressure, and chemical changes over millions of years. Originally, this rock started as mudstone or shale, deposited as fine sediments in ancient seas over 400 million years ago during the Cambrian and Ordovician periods.During the Caledonian Orogeny (a mountain-building event), these sedimentary rocks were deeply buried and subjected to intense heat and pressure. This caused the minerals to recrystallize and align into thin layers, giving slate its unique foliation—a property that allows it to split into thin, uniform sheets.
What does metamorphism mean for slate?
- Increased Hardness & Strength – The heat and pressure compacted the rock, making it stronger and more durable than its original form.
- Low Water Absorption – The dense structure prevents water from seeping in, making slate resistant to frost and weathering.
- Cleftability – The mineral alignment allows it to be split into thin, even sheets, making it ideal for roofing and construction.
However, during this process, impurities such as calcite and quartz could be introduced, affecting the slate’s texture, durability, and visual appearance. Welsh slate is often considered some of the world's best due to its reputation as a low-impurity stone, but the slate here seems to be an exception to this rule.
Impurities in Slate: Calcite vs. Quartz
As fluids moved through the rock during metamorphism, different minerals were introduced, forming impurities that affect the slate's properties. At this location, you may observe two of the most common ones:
Calcite Impurities (CaCO₃) – Soft & Weakening
- Appearance: White or light-colored veins or streaks, often smooth or chalky-looking.
- Hardness: Soft (3 on Mohs scale)
- Effect on Slate: Weaker than the surrounding rock, making the slate more prone to cracking and weathering.
- Formation: Introduced by carbonate-rich fluids that moved through the rock during metamorphism.
Calcite veins can reduce the durability of slate, making it less ideal for roofing or construction. Slates with high calcite content are often avoided for long-term structural use.
Quartz Impurities (SiO₂) – Hard & Strengthening
- Appearance: Light-colored or translucent patches, often crystalline or rough in texture.
- Hardness: Very hard (7 on Mohs scale)
- Effect on Slate: Strengthens the rock, making it more resistant to weathering but harder to split.
- Formation: Formed when silica-rich fluids infiltrated the rock during metamorphism.
Quartz impurities enhance the durability of slate, making it more resistant to wear. However, if present in large veins, quartz can make slate difficult to split and less workable for construction.
Questions
To log this EarthCache, please send the answers to the following questions by email or message. Due to owning many EarthCaches I receive numerous emails per day so there is often a delay in replying, therefore feel free to log before awaiting a reply. Only answers that are drastically incorrect or photos that reveal answers will be deleted.
1. What is the colour and texture of the slate in this wall? Is the impurity the same colour/texture, or different?
2. Based on your observations and the information on this cache page, do you think the impurities here are quartz or calcite?
3. How does this impurity affect the structure of the wall?