Introduction
The rock you see here in the layers exposed by the road cutting, is a very common type of rock, but few people know much about it, and I'm hoping you'll learn something about it too. I'll try to keep it simple mainly for the younger ones.
How was it formed?
A very very long time ago, an accumulation of mud began with the chemical weathering of rocks. This weathering broke the rocks down into clay minerals and other small particles which became part of the local soil. Many rainstorms washed the tiny particles of soil from the land into streams, giving the streams a "muddy" appearance. When the streams slowed down or entered a standing body of water such as a lake, swamp or ocean the mud particles settled to the bottom. Undisturbed, it was buried and this accumulation of mud was transformed into this sedimentary rock here, known as "Shale". This is also how most other sedimentary rocks are formed.
Classification
For consistency in classification, sediment is divided by particle size into gravel , sand, silt and clay, with clay having the smallest particles, smaller than 1/256mm. To be classified as claystone, it has to contain at least twice as much clay as silt and no more than 10 percent sand.
Shale is claystone that is fissile, meaning it splits in layers with relative ease. It is this splitting characteristic (or fissility) that distinguishes it from ordinary claystone which is solid and not able to split.
Shale can be fairly hard if it contains silica cement, but usually it is soft and easily weathers back into clay. Because it is soft, it does not crop out and may be hard to find except in roadcuts like here, unless it is protected from erosion by another harder rock.
When shale undergoes greater heat and pressure, it becomes the metamorphic rock slate.
Colors
Like most other rocks, the presence of just a small percentage of specific minerals, iron or organic material can significantly change the color of the shale.
Black or dark grey indicates the presence of organic materials, as little as just one or two percent at formation. This also implies an oxygen-deficient environment, since oxygen would have reacted to the organic materials and decayed it away.
Red, Brown and Yellow Shale deposited in oxygen-rich environments often contain iron oxide or -hydroxide minerals. Just a small amount these minerals distributed through the rock can produce the red, brown or yellow colors visible in many types of shale.
Gray shales could contain a small amount of organic matter but just like green shale, can also only be containing clay minerals resulting in a gray, or greenish color.
Uses
Certain shales have special properties that make them very useful such as black shales that can contain natural gas or oil, or other shales that can be crushed and water-mixed for the production of a variety of useful objects, that were traditionally made from natural clay. Bricks, “terra-cotta” pots and floor tiles are examples of products now manufactured from shale due to the depletion of natural clay deposits.
Shale is also used in the production of cement for the construction industry by crushing limestone and shale and then heated to a high temperature. This evaporates all water and breaks down the limestone into calcium oxide and carbon dioxide. The calcium oxide combines with the heated shale to make a powder that will harden if mixed with water and allowed to dry.
Stability
Weathering transforms shale into a clay-rich soil which cause a variety of problems with construction or otherwise. On slopes, landslides are caused by failure due to very low shear strength, especially when wet. This clay-rich soil also expand when wet, shrink when drying, causing immense forces in the soil, cracking and weakening buildings erected with improper foundations.
To claim this find, please answer the following questions by email to the cache owner. All logs not accompanied by an email, will be deleted. Please do not put any answers in your log.
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Look at the layers of shale that is visible in the road cutting, what colours do you see?
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Estimate the angle of the layers, in degrees, relative to the road.
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In a few words, what have you learned, or found interesting while visiting this site?
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If you wish, you may share a photo, but is optional and not required.