Report by Mihaela Bokor
On Sunday 25 May, a group of 18 intrepid budding and professional geologists gathered in Royal Leamington Spa for an eye-opening Geo Walk, led by geologists Jane and Mihaela. This wasn’t just any urban stroll—it was a guided exploration through time, where building stones became storytellers, whispering tales of ancient seas, fiery magmas, and shifting continents.
Over the course of the morning, the group journeyed through the town centre, using a digital microscope to examine the fine textures, minerals, and fossils hidden in plain sight on building façades, paving stones, and decorative columns. Each stop along the route uncovered a different rock type, as Jane and Mihaela explained their origins, mineral compositions, and the environments in which they formed—often hundreds of millions of years ago.
First there was a bit of background provided by Jane about the geological origin of the springs that lead to the ‘Spa’ in Leamington Spa, the springs essentially forming at the interface of the Bromsgrove Sandstone (now Helsby Sandstone) or the Triassic and the also Triassic Mercia Mudstone Group. Many of the buildings in Leamington Spa are not constructed of load bearing decorative stones but rather from bricks made from clays mined from the local Mercia Mudstone which is then rendered to look like Portland Limestone.
The tour included in depth observations and discussions on:
Sedimentary Rocks: Layered Histories
The first chapter of our walk began with sedimentary rocks, formed by the gradual accumulation of sediments laid down in ancient rivers, deserts, and seas.
- Feldspathic Sandstone Columns (The Pump Rooms) with distinct Liesegang rings were an early highlight. These rings, formed by rhythmic precipitation of iron oxide during diagenesis (the rock’s formation process), offered beautiful concentric patterns—almost like tree rings frozen in stone.
The sandstone’s high feldspar content indicates limited chemical weathering and proximity to granitic source areas (Blatt et al., 1980).
- Helsby Sandstone, a Triassic red-bed sandstone (~240 million years old), featured prominent cross-bedding structures, hinting at ancient dune or river environments. These sedimentary layers told of shifting currents and wind-blown sands in what was once a semi-arid desert landscape.
- Pennine Sandstone paving slabs, coarse-grained and durable, represented ancient deltaic deposits. These stones, quarried from the Pennines, Yorkshire, are commonly used in northern architecture and give towns like Leamington their robust streetscape. A Carboniferous-aged sandstone laid down in deltaic to coastal settings. Common in building and paving across the UK (British Geological Survey, 2021).
- Several iron-stained sandstones were also noted, their warm orange and reddish hues created by oxidation of iron minerals—a common but striking transformation over geological time.
- The group examined two classic oolitic limestones:
- Cotswold Limestone, formed during the Jurassic (~170 million years ago) in warm, shallow seas, composed almost entirely of ooids—tiny spherical grains formed by rolling in calcium-rich waters. (Town Hall).
- Portland Limestone from Dorset (Hotel Chocolat), also oolitic but often packed with fossilised oyster shells, giving glimpses of marine life that once thrived in the Jurassic seas. This stone is famously used in buildings such as St Paul’s Cathedral.
- Travertine, a banded, porous limestone formed from calcium carbonate precipitated by spring waters, was seen in some decorative claddings, often associated with geothermal or CO₂-rich springs (Pentecost, 2005). Its formation is still happening today in places like Italy’s hot springs—making it a living counterpart to other rocks examined.
Metamorphic Rocks: Recrystallised Beauty
As the walk continued, we moved from sedimentary to metamorphic rocks—those transformed by heat, pressure, or chemical fluids deep within the Earth.
- Vein Marble, with its intricate, swirling patterns, told the story of mineral-rich fluids infiltrating cracks in existing rocks, then crystallising under pressure to form visually striking veins—often composed of calcite or quartz.
These rocks reminded us that metamorphism not only changes structure but often enhances a stone’s visual appeal—making it a favourite for high-end finishes.
Were we saving the best until last? Possibly because before we finished at Jephson gardens where we examined some beautiful igneous rocks. Michaela explained that the size of the clasts in the rocks were dependent on a number of factors such as: long crystallisation time (feature), stable conditions at depth (cooling rate), ample chemical components (crystal size) of the magma. Using the chart below us moved through Granite to Granodiorite and Gabbro rocks with the associated minerals.
Igneous Rocks: Crystallised from “Fire” (plutonic rocks)
The tour culminated with a closer look at igneous rocks, born of molten magma and shaped by the rates and depths at which they cooled.
The group examined several types of granite:
- A warm-hued pink Scottish granite from Aberdeen, with large, visible crystals of feldspar, quartz, and mica.
- Equigranular granite, where al minerals are roughly the same size, indicating slow and uniform cooling deep underground.
- A striking porphyritic granite, containing large feldspar phenocrysts embedded in a finer matrix. These phenocrysts displayed a perthitic texture, where exsolution of potassium and sodium feldspar creates distinctive stripy intergrowths—visible under the digital microscope.
We also looked to a gabbro underneath of the travertine in front of the McDonalds and the first inspection showed to us that is a mafic intrusive igneous rock, forms when mafic magma cools slowly deep in the Earth’s crust. Contains less silica that the other igneous rocks we have been looking and are rich in magnesium and iron rather than the feldspar and quartz (as you can see on the chart)
Particularly eye-catching were the Larvikites. There are two varieties of this Norwegian igneous rock:
- Blue Larvikite (Marks and Spencer) is an intrusive igneous rock; is predominantly composed of feldspar minerals, particularly ternary feldspars (alkali, anorthoclase, perthitic) and with a dazzling Schiller effect caused by light reflecting from aligned feldspar crystals.
- Emerald Pearl Larvikite, dark with shimmering flecks of green and silver, often used in polished countertops and upscale exteriors.
These rocks not only reflect geological processes but show how aesthetics and durability influence architectural stone choices.
After inspecting more limestones and sandstone in Jephson Gardens we finished on the bridge with the cross bedded Helsby Sandstone. This Geo Walk showcased how urban architecture acts as a geological museum – each rock block in the wall or pavement can be a window into ancient environments. Cross-bedding in sandstones hinted at past wind-blown dunes or river channels, while the sparkle of feldspar crystals connected modern structures to magmatic processes deep within the Earth.
Learning Beyond the Surface
Throughout the tour, Jane and Mihaela shared not just facts, but enthusiasm—encouraging participants to observe, touch, and question. With the help of digital microscope, textures normally overlooked—oolitic grains, feldspar twins, fossil shell fragments—were revealed in fascinating detail.
Participants were often surprised to realise how diverse and ancient the rocks around them are. Stones used in ordinary building façades may have formed hundreds of millions of years ago in environments as varied as tropical seas, or deep underground magma chambers.
Final Thoughts
The Geo Walk offered more than just geological facts—it invited people to see their town in a new light. Leamington Spa, like many urban environments, is a living geology museum, where stones from across the UK and the world come together in a built landscape.
A massive thanks to Ray for all his support during this tour (Geo walk) and his time before to help us to do a good prepare and understanding. Another thank you goes to Kathrin for joining us during the tour and taking lovely photos you will see them below.
Time for a well-earned ice cream then!
References
- used the excellent Warwickshire Geological Conservation Group leaflet as a guide to our walk;
- BGS – https://webapps.bgs.ac.uk/Memoirs/docs/B01616.html
- Blatt, H., Middleton, G., & Murray, R. (1980). Origin of Sedimentary Rocks (2nd ed.). Prentice Hall.
- Ian Sanders(2018)- Introducing metamorphism;
- John Mason (2015)– Introducing mineralogy;
- Pentecost, A. (2005). Travertine. Springer.