The anatomy of a wall....Slickensides. EarthCache

🚨You are best walking to the EarthCache location. The road from Helwith Bridge, is often busy with quarry waggons🚨. 

In geology people tend to look for the larger features, as they are easier to see, though sometimes smaller things can catch the eye, and then lead to interest and understanding. In some locations, looking at the rocks which make up the anatomy of a wall can give interest, and also learning about geology. The below photograph shows a wall in nearby Little Stainforth, which demonstrates the geology of the local area. 

(Photograph: Hillgorillas' wall).

We are here to look at an aspect of geology on a macro level. You would benefit from having a camera📷 or mobile phone🤳 that can take an image on a macro level, or a magnifying glass🔎. The below photograph shows where to look. DO NOT MOVE ANY STONES. 

The geology of the wall.

The wall is made up of greywacke, which was deposited during the Silurian period, which was 445 to 420 millions of years ago. Geologically, its story starts in an ancient ocean known as the Iapteus Ocean. Sediment in the form of sand and silt was laid down on the ocean floor, then turbulent movement of the sea bed occured. Over time sediment built up , and eventually rock was formed. However this is not the end of the story. The Iapteus Ocean dissapeared eventually, and the sedimentary layers were uplifted and folded due to massive tectonic forces. The rock was also subjected to metamorphism, which is where pressure and heat alter the structure of the rock.

So, what are we here to see?

We are here to look at a geological feature known as slickensides. 

What are slickensides?

A slickenside is a smooth polished surface. A slickenside is produced by the friction caused by two rocks coming together along the two sides of a fault line. Striations are lines, a bit like a stretch mark is a striation, but instead of being caused by a baby, here the striation is caused by the movement and friction along the fault line. 

The striations  are due to friction, which we have all encountered it in one sort of way in our lives, be it our heels rubbing against a shoe causing a blister, the brakes on our bicycle wheels causing them to stop, or when we have an arguement with our partner . Friction also occurs in the world of geology, and that is why we are here. Don't worry though, this EarthCache does not involve having to complete a thesis on the subject.  I have brought you here to learn, and see evidence of a geological process. As human beings, we all have the ability to move in one way or another. The rocks that form our planet also move at times, allbeit, not as quick as a human walking. Rocks can and do, move against each other along what are known as fault lines.Though if you look at the rocks around you, and compare yourself to them, you will find that you are a flexible being, whilst the rocks are a rigid static being.  For this reason, when two sides of a fault line rub against each other, then friction is formed. That friction creates strain and stress, and also at times physical evidence of it happening.

Why are they here?

Well, the rock has formed part of the wall because it has been quarried nearby. As stated above the geology has been subjected to a lot of stress due to metamorphism, and also due to tectonic forces affecting the rock. Basically, the rock layers have been moved from the horizontal to now being folded . This has meant there has been movement in the rock. The area is also near to the North Craven fault system, where a series of faults or cracks have formed, which then contribute to stress and friction within the rock. 

This being an EarthCache, in order to log it, I ask that you answer the below questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.

1. Please describe the specific rock in terms of structures, feel and colour. 

2. Please explain in your own language how the slickenside has formed.