The Withnell Fold War Memorial is more than a site of remembrance—it’s also a showcase of geology, frozen in stone. This EarthCache focuses on the granite used in the memorial’s base and upright column, particularly the phenocrysts visible in the rock. These large, reflective mineral crystals formed deep underground and now sparkle in the sunlight at the surface.
Granite is an igneous rock that forms when molten magma cools slowly beneath the Earth's crust. Because of this slow cooling, crystals have time to grow, often forming visible mineral grains known as phenocrysts. These are noticeably larger than the surrounding finer-grained matrix and give granite its characteristic “salt and pepper” or speckled appearance.
At the Withnell Fold War Memorial, you’ll be able to observe these crystals closely. As you study the stone, you’ll uncover the geological processes that shaped it millions of years ago—and the minerals that still glint in the Lancashire sun today.
What Are Phenocrysts?
Phenocrysts are large, well-formed crystals that developed earlier than the surrounding rock during slow magma cooling. In granite, the most common minerals forming phenocrysts include:
- Quartz – typically grey or glassy; equant in shape
- Feldspar – white, pink, or grey; tabular or blocky
- Mica (biotite or muscovite) – black, shiny flakes; often anhedral
Common phenocryst shapes include:
- Euhedral – well-formed with clear crystal faces
- Subhedral – partially formed crystal faces
- Anhedral – irregular, no clear crystal shape
- Tabular – flat and plate-like (common in feldspar)
- Equant – equal dimensions in all directions (common in quartz)
Logging Tasks:
To log this EarthCache, visit the memorial and send your answers to the following questions via the Message Center or email. Do not post answers in your log.
- Phenocryst Observation:
a) Can you see large mineral crystals (phenocrysts) in the granite?
b) Describe their appearance: What color are they? Are they glassy, shiny, or dull?
c) Based on their shapes, classify them (e.g., tabular, equant, subhedral, anhedral, etc.).
- Mineral Identification:
a) Which mineral do you think is causing the sparkle or reflective surfaces in the granite?
b) What color is this mineral, and what does that suggest about its identity (e.g., quartz, feldspar, mica)?
- Formation Conditions:
What does the size of the phenocrysts tell you about how the granite formed? (Hint: Think about cooling speed and crystal growth.)
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***** PLEASE NOTE IMPORTANT *****
CACHES ARE NOT ALLOWED TO BE PLACED ON ACTUAL MEMORIALS OR WITHIN THE BOUNDARY OF SUCH
AT ALL TIMES PLEASE TREAT LOCATIONS OF MEMORIALS WITH RESPECT