Welcome to Regent’s Park
At Regent’s Park in London, a remarkable group of fossilized tree stumps and algal burrs offers a window into Earth’s deep past. These ancient conifers once grew along the margins of hypersaline lagoons during the Late Jurassic period, over 145 million years ago. This EarthCache invites you to explore these rare and beautifully preserved fossils, learn about how they formed, and understand their geological significance.
The Fossilization of the Purbeck Trees
The fossil trees in Regent’s Park are the silicified stumps of Protocupressinoxylon purbeckensis, an extinct conifer species that lived around 147 million years ago during the Tithonian stage of the Late Jurassic. These trees once grew in a harsh environment, on the margins of shallow, hypersaline lagoons in what is now southern England. The soil was thin and rich in lime, formed atop older layers of marine limestone. Repeated cycles of seawater flooding and evaporation caused calcium carbonate to build up in the ground, creating a hardened, fossil-bearing soil horizon called a palaeosol. These ancient soils preserve vital clues about past ecosystems and climate conditions.
The tree stumps were originally rooted in a geological layer known as the Great Dirt Bed, part of the Mupe Member within the Lulworth Formation of the Purbeck Group. This layer is especially rich in fossils and biological textures, preserving plant roots, leaf litter, and microbial structures. As the trees died and became buried under sediment, mineral-rich groundwater slowly flowed through the area. Over time, silica from the groundwater gradually replaced the wood’s organic material in a process known as silicification. This type of fossilization preserved the tree stumps in exceptional detail, right down to the growth rings, bark patterns, and microscopic wood cells.
Scattered near the fossil stumps are distinctive ring-shaped carbonate formations called algal burrs. These formed when cyanobacteria and algae thrived at the base of the trees, precipitating calcium carbonate from the lagoon’s saline waters. As the microbes grew, they deposited the mineral in concentric layers around the tree bases, eventually creating ring-like thrombolites with clotted internal textures. Unlike the more orderly layering of stromatolites, thrombolites have a chaotic internal structure that reflects irregular microbial growth.
The stumps now on display were originally quarried in Dorset during the 19th century by workers extracting Portland Stone. Some were acquired by museums and private collectors, while others were placed in public gardens as natural ornaments. The ones in Regent’s Park were a gift from the Royal Botanical Society, sometime between the late 1800s and early 1900s. Their current home in the Japanese Garden offers visitors a rare chance to see Jurassic fossils in an outdoor setting and to imagine the ancient landscape they once inhabited.
Tasks for This EarthCache
To log this EarthCache, visit the site in Regent’s Park and complete the following tasks. Send me your answers via Geocaching or email.
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Include “Fossil Trees - Regent’s Park - GCB7D8D” on the first line of your message.
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Describe the color and texture of one of the fossilized tree stumps. Is the grain or bark structure still visible? What does this tell you about the permineralization process?
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Locate one of the algal burrs. What shape is it? How would you describe the texture of the algal burr compared to the fossil tree stump? Explain why it might be similar or different.
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Do you see any cracks, chips, or discoloration on the fossils? What natural or human-related factors might contribute to these changes?
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In your log, attach a photo of yourself or a personal belonging with an algal burr in the background. (Note: photos predating the publication of this EarthCache are not accepted.)
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