The Tower of St. Elsyng Spital
St. Elsyng Spital, located on London Wall, has a fascinating history that dates back to medieval London. Founded in 1330 by William Elsyng, a wealthy merchant, the site was originally established as a hospital to care for the city's poorest residents, including the homeless and blind.1,2. It was more akin to an almshouse, providing shelter and spiritual care rather than medical treatment.3
In 1340, the hospital evolved into a priory church, staffed by Augustinian Canons. William Elsyng himself passed away during the Black Death in 1349 and was buried in the church.2 The site continued to serve the community until the Dissolution of the Monasteries in 1536, after which much of the hospital was repurposed into private residences.1,2
Over the centuries, the site underwent significant changes. It became part of Sion College in 1630, suffered damage during the Great Fire of London in 1666, and was rebuilt multiple times following destruction during World War I and World War II.1,3 Today, only the tower of the original church remains, standing as a historical landmark amidst modern developments.1,3
Although little remains of St Elsyng Spital today, studying its remnants reveals how medieval builders relied on local geological resources and adapted them for construction. Observing these materials also offers insights into the geology of the region and the resourcefulness of past societies.
Flint
Flint is a fascinating sedimentary rock that forms a unique intersection between biology and geology, embodying complex chemical and physical processes that have unfolded over millions of years. Essentially, flint is a cryptocrystalline form of quartz (silicon dioxide) that is most commonly found within chalk and marly limestone deposits. Its formation is a splendid example of diagenesis — the process by which sediments are lithified and altered chemically after their deposition.
Biological Contributions and Opaline Beginnings
At the heart of flint’s formation lies the contribution of siliceous organisms. In ancient chalk seas, organisms such as sponges, diatoms, and radiolarians actively collected silica from seawater to build their skeletons or cell walls. When these organisms died, their remains—composed largely of what is known as biogenic opal—settled onto the seabed. Over time, as more sediment accumulated, these microscopic silica particles became incorporated into the sediment matrix. At relatively shallow sediment depths (typically around 1 to 5 meters), the biogenic opal began to break down, releasing silica into the pore waters within the sediment. This process laid the foundation for the eventual formation of flint as the dissolved silica became available for further chemical reactions.4,5
Diagenetic Processes and Chemical Transformation
The critical step in flint formation occurs during diagenesis, where the chemical environment of the sediment plays a pivotal role. As sediments accumulate, they are subjected to increasing pressures and burial over geological time. In these conditions, fluctuations in pH and the presence of various ions in the pore water encourage the precipitation of silica. In particular, an important factor is the local decrease in pH that occurs at the oxic-anoxic boundary within the sediment. Here, hydrogen sulfide—produced as organic matter decays—reacts with oxygen to produce hydrogen ions that help dissolve portions of the chalk. The resulting chemical milieu enriches the pore water not only with carbonate ions from the chalk but also with silica released from the breakdown of biogenic opal. The dissolved silica gradually begins to precipitate, molecule-by-molecule replacing the chalk matrix and eventually forming nodules of microcrystalline quartz, which we recognize as flint.4,5,6
The Role of Sedimentary Structures
Another intriguing aspect of flint formation is the influence of the original sedimentary structures, such as the burrows and cavities left by ancient marine animals. These spaces often serve as preferential sites for the precipitation of silica. The irregular and sometimes nodular shapes of flint nodules are thought to reflect the remnants of such burrows or cavities, where the conditions for chemical reactions might have been particularly favorable. Additionally, the layered or banded appearance of flint in some cases may result from cyclic variations in sedimentation, with episodes of high silica concentration interspersed with periods where the process essentially resets when conditions become optimal again for silica deposition.5
From Ancient Seas to Human Innovation
Over time, the initially opaline silica undergoes recrystallization under increasing pressure and temperature, transforming into the highly durable and glassy quartz that is flint. This transformation is critical—it is the reason behind flint's famed conchoidal fracture, which produces extremely sharp edges when broken. These properties made flint indispensable to early humans, not only as a tool-making material for cutting and hunting, but also as a means to generate fire through spark production. Thus, flint is not just a geological marvel but also a witness to the technological ingenuity of our ancestors, who mastered its properties for survival and innovation.4,6
In sum, the formation of flint is a multifaceted process involving the deposition of biogenic silica in ancient marine environments, its chemical transformation during diagenesis under varying geochemical conditions, and the structural influence of sedimentary features. This transformation encapsulates millions of years of Earth’s history — a history that links the evolution of marine life with the material culture of early human societies.
Flint at St. Elsyng Spital, a closer look.
Flint's color variations are primarily influenced by mineral impurities present during its formation. Different elements contribute to its hues: 1) Iron Oxides – These impurities give flint a red, brown, or yellow tint. The oxidation of iron within the rock leads to these earthy tones. 2) Organic Material – Flint that contains remnants of ancient organic matter often appears black or dark gray. This is common in flints found in chalk deposits. 3) Calcium and Magnesium – These minerals can lighten flint's color, resulting in white or pale gray varieties. 4) Manganese and Other Trace Elements – Occasionally, flint can exhibit blue or greenish hues, which are attributed to trace amounts of manganese or other minerals.
Flint's formation process involves silica precipitation within sedimentary rocks like limestone or chalk, and the presence of these impurities during this process determines its final coloration, which in turn influenced its applications.7,8
Wheatering of flint.
Flint is subject to several weathering processes that alter its appearance, structure, and even its utility in archaeological interpretations.
Patination and Surface Alteration One of the most visible effects of weathering on flint is the development of a white patina. This occurs when atmospheric or groundwater agents leach out the amorphous silica that originally fills the spaces between the individual quartz crystals. As these components are removed, microscopic cavities form that scatter light, resulting in the characteristic white, sometimes chalky, appearance. This phenomenon, first detailed by Judd in the late 19th century and elaborated upon by later researchers, provides important clues about the flint's exposure to its environment over time.9
How to claim this EarthCache?
Send me the following;
1. The text "GCB74RA The Tower of St. Elsyng Spital on the first line.
2. The answers to the following questions;
- What is the primary geological composition of flint?
- What does the presence of flint in buildings tell us about the geology of the surrounding area?
- Observe the colour(s) of the flint used here, which impurities caused this?
- Can you find any visible signs of weathering?
3. Take a selfie (optional) and/or a photo of a thumbs-up, peace-symbol (V) or personal item, clearly showing the tower of St. Elsyng Spital in the back, and attach it to your log.*
References
1 julianwhite.uk, Central London – Tower of St Elsyng Spital. 2 patrickcomerford.com, Elsyng Spital and the sites of two mediaeval churches in the City of London (Original 01/2020). 3 theunfinishedcity.co.uk, Elsyng Spital Church Tower, City of London (Original 02/2025). 4 Wikipedia, Flint. 5 http://snpr.southdowns.gov.uk/, David Bone How flint is formed. 6 geologyscience.com, Flint (Modified 02/2025). 7 rockhounding.org, Flint. 8 stonehards.com, Flint: A comprehensive guide. 9 cambridge.org, Flint and the patination of flint artefacts. 1-9 (Retrieved 05/2025).
* Effective immediately from 10 June 2019, photo requirements are permitted on EarthCaches. This task is not optional, it is an addition to existing logging tasks! Logs that do not meet all requirements posed will no longer be accepted.
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