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An Earthcache examining a slow acting geological process and a device to measure it. Please note an entrance fee is required to visit the Earthcache.
The Victorian naturalist Charles Darwin is best known for his theory of evolution by natural selection, which has transformed our knowledge of life on the planet. What is less well known is that Darwin's original interest was Geology, and during his career he published works on a diverse range of topics including coral reef structure and the formation of volcanic islands.
Darwin's journey around the world on the HMS Beagle collecting specimens and making observations of the variations amongst species was instrumental in the formation of his ideas, but his real laboratory was his home and garden. It was here that Darwin made investigations into an easily overlooked geological process: the action of worms on the geology of the Earth's surface.
Darwin's interest in the action of earthworms was first triggered by a visit to his uncle, Josiah Wedgwood II in 1837. Whilst strolling through a field, Wedgwood remarked that 15 years earlier the surface had been covered with lime, ash and marble, which was now buried. His uncle stated the belief that earthworms were responsible. Related correspondence between Wedgwood and Darwin can be viewed at the Darwin Correspondence Project.
Darwin later scattered rocks across a meadow to the south east of Down House, "for the sake of observing at some future period at what depth it would become buried". Thirty years later Darwin observed that the rocks had sunken some 18 cm into the ground. This observation was also made for fallen stones at ancient sites such as Stonehenge. The following figure is taken from Darwin's publication on the subject, "The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits"
An elegant apparatus, called the wormstone, was designed and constructed to aid in the measurement of the sinking of stones relative to the surface of the ground. It was constructed by laying a mill stone on the ground and driving metal bars through the hole in the centre and into the bedrock. The subsidence of the stone over time relative to the top of the metal bars was measured by a 'Wormograph' (constructed by Darwin's son Horace in 1870 and on display on the first floor of Down House) that sat on top of the stone to measure the height difference between the stone and the rods. The change in the height indicated the action of worms on the soil between the stone and the bedrock. The wormstone located at the above coordinates has subsided considerably over the years. There are conflicting reports whether the stone in place today is a replica, or the relocated original.
The phenomena of soil mixing by organisms is now known as bioturbation, and is considered an important process in both terrestrial and aquatic environments. The action of the organisms engineers their environment, enhancing their access to nutrients and changing the microbial flora. Bioturbation may also have played a key role in the evolutionary "Cambrian explosion" of diversity of species, as the working of the soil provided new avenues to avoid predators or chase prey.
To claim this Earthcache you must complete the following tasks:
1. Approximate the depth the wormstone has sunk from the top of the metal bars.
2. Approximate the width of the mill stone used in the construction of this apparatus.
3. In recent years the wormstone has stopped subsiding and is unlikely to sink further. Please give an explanation for why this has occurred.
4. Name one animal other than the worm implicated in bioturbation.
5. (Optional) Take a photo of yourself with your GPS with Down House in the background and upload it to your log.
Please email me the answers through my profile link prior to logging the cache.
The Earthcache is located within the grounds of Down House, protected by designations for landscape and biodiversity, and an entrance fee is required to visit the property and complete this cache. Limited parking is available at the listed additional waypoint and visitors are encouraged to make use of local bus services. Please see either the Transport for London website, or the "Directions" tab of the Visitor Information website.
This Earthcache has been developed and placed with the kind assistance and permission of English Heritage. Information about the house and gardens can be obtained from their website www.english-heritage.org.uk/downhouse.
Arne gur purfgahg gerr.