Welcome to my Earthcache!
An EarthCache is a special type of geocache where there is no container to find. Instead, you explore a unique geological feature and answer questions to claim your find, along with posting a photo. This EarthCache is explores urban decay and weathering of limestone, what factors may contribute to this particular type of weathering, and how limestone is particularly prone to "black crust" weathering. All observations can be made from the sidewalk at the posted coordinates, where you will find a stone pillar remaining from the 1860's estate of Dan Rice.
EarthCache Requirements
As with all of my EarthCaches, I’m not expecting PhD-level answers. Take some time to enjoy the site and learn something new. If you’re answering for multiple caching names, include a list of cachers with your answers so I can verify that you have completed the requirements for this Earthcache.
To log a find, please send your answers to the following questions via the link at the top of the page or email the owner:
Questions to Answer
- Describe what the black crust looks like (for example, thin stain, thick crust, patchy, smooth, or flaky).
- Find an area where the stone has experienced some damage, with the original stone colour visible in the cracks or in an area that has broken away. Approximately how thick is the crust on the pillar? Do you think it is continuing to accumulate, or are there signs that it has stopped growing?
- Compare a sheltered area of the stone (such as under a ledge or overhang) with a more exposed vertical surface. Which part shows more black crust, and which looks cleaner? Suggest a reason for this difference.
- Using the information in the description as well as your observations, explain in your own words why limestone in a polluted urban environment is particularly prone to developing black crusts.
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Photo Requirement: Take a photo of the yourself, a signature item, geo-pet, or trackable item in front of the pillar at GZ. Your face is not required to be in the photo but it should be attached to your log.
You may log your find once you have sent your answers. I will contact you if anything needs clarification.
Geology Lesson
The stone pillar in front of you is one of several that surrounded the original estate of Dan Rice, as part of the wall that surrounded the home circa 1860. This wall was built with local materials, including limestone. Limestone is a sedimentary rock that may look solid and unchanging but actually reacts with the atmosphere around it. In cities, sandstone is exposed to polluted air and rainwater, which can gradually alter its surface and create a dark coating known as black crust.
At this EarthCache, you will observe how black crust forms on the limestone, where it is most visible on the building, and how it relates to chemical weathering and stone decay. This is an example of geology and geochemistry at work in an urban setting.
What Is Black Crust?
Black crust is a dark, often grey‑black layer that develops on stone surfaces, especially in polluted towns and cities. It is common in areas where mills and factories have lead to an increase in air pollution - especially near coal mines and gas refineries. It is particularly common on calcareous stones like limestone and marble, but it also forms on sandstone, especially where the stone contains carbonate-rich cement or other reactive minerals.
This crust is not just dirt stuck to the surface. It consists mainly of new minerals (most often gypsum, a calcium sulfate), together with trapped soot and dust from the air. The chemical reactions that produce this layer take place at the interface between the stone, the atmosphere, and thin films of moisture from rain or humidity. As a result, black crust is considered a product of chemical weathering of building stone, rather than a simple deposit.
How Does Black Crust Form?
In an urban environment, the air can contain pollutants such as sulfur dioxide from burning fuels, nitrogen oxides from vehicles, and fine particulate matter like soot and dust. When these gases dissolve in rainwater or in films of moisture on the stone surface, they form weak acids. This is a key part of what is often loosely called acid rain.
When acidic moisture reaches the stone, it can react with minerals in the sandstone and especially with carbonate-rich cements. Calcium from the stone can combine with sulfur from the pollution to form gypsum (CaSO₄·2H₂O). This gypsum forms near or at the surface and can create a slightly roughened or crusty layer.
This new mineral layer is sticky and porous. It tends to trap airborne particles such as soot and dust, which darken the surface and give the crust its black or dark grey colour. Over time, repeated cycles of wetting, reaction, and drying allow this crust to become thicker and darker, particularly where it is not washed away.
Why Is Black Crust Often Thicker in Sheltered Areas?
On many buildings, black crust is more developed in places that are sheltered from direct rain, such as under window sills, projecting cornices, carved decorations, and recesses. These spots still receive polluted air and moisture, so chemical reactions can occur, but they are not regularly cleaned by heavy rain running down the surface.
Exposed wall faces, which are more directly hit by rain, may still weather chemically, but any loose gypsum and trapped particles are more easily washed away. As a result, the contrast between sheltered, blackened zones and relatively cleaner, exposed areas is often quite obvious on older buildings in cities.
This pattern is something you can observe directly at this location and use to infer how water, air pollution, and stone interact on different parts of the façade.
Why Limestone Is Susceptible to Black Crust
Limestone is composed primarily of calcium carbonate (calcite), a mineral that readily reacts with acidic pollutants in the atmosphere. In urban and industrial environments, sulfur dioxide and other airborne pollutants dissolve in rainwater or surface moisture to form weak acids. When this acidic water comes into contact with limestone, it reacts with the calcite at or near the stone’s surface.
This chemical reaction converts calcium carbonate into gypsum (calcium sulfate), which is more soluble and tends to crystallize on the stone’s exterior. Gypsum is particularly effective at trapping airborne soot, dust, and other pollutants, giving rise to the dark, hardened layer known as black crust. Because limestone often has a relatively fine-grained texture, this crust can form as a distinct surface layer rather than being evenly distributed through the stone.
Over time, the growth of gypsum crystals within surface pores and microfractures creates internal stress. This can lead to flaking, scaling, and detachment of the crust, often removing the underlying limestone with it. As a result, black crust on limestone is not just a cosmetic issue but a clear indicator of ongoing chemical weathering and surface deterioration.
Black Crust and Stone Decay
Black crusts are often associated with areas where the stone is visibly deteriorating. Thick crusts may develop on surfaces that are already retreating, and, in some cases, the crust can detach and take the outer skin of the stone with it. This process can be seen on sculptural elements and fine architectural details, where the original shapes become softened or lost.
By examining the sandstone slabs at GZ, you can see that the development of black crust is not just a cosmetic issue. It is closely linked to the long-term durability of sandstone and helps illustrate how human activity and natural processes combine to change even apparently solid stone in the built environment.
References
Doehne, E., & Price, C. A. (2010). Stone Conservation: An Overview of Current Research. 2nd ed. The Getty Conservation Institute.
Brimblecombe, P. (1992). “Urban air pollution and historic buildings.” In The Effects of Air Pollution on the Built Environment, edited by P. Brimblecombe, Elsevier, 1–30.
Fitzner, B. (2002). “Damage diagnosis on stone monuments—weathering forms, damage categories and damage indices.” In Stone Decay and Conservation, Geological Society, London, Special Publications, 205, 11–56.
AI Content Disclosure: Some of the descriptive text was created with assistance from AI tools. All information has been reviewed and verified by the cache owner for accuracy.
This cache was placed by a PROUD Platinum EarthCache Master.
