THIS IS A HIGH TERRAIN EARTHCACHE
IT INVOLVES SOME EXPOSURE ABOVE A ROCKY DROP, THEN A SCRAMBLE DOWN TO THE STREAM BED AND BACK UP AGAIN
IT IS ROUGH UNDERFOOT
ASSESS THE RISK FOR YOURSELF
THERE ARE TWO STAGES TO THIS EARTHCACHE
THERE ARE STEEP CLIFFS
The EarthCache co-ordinates take you to Backstone Beck, to a narrow spit of rock which is like a thumb and forefinger.This is known as Rombalds Thumb.The thumb and forefinger sit just above a waterfall, above the narrow gorge of the beck.
Stage One
Standing on the thumb you are standing in a geological fault line.
What is a Fault Line?
A fault is a fracture or discontinuity in a volume of rock, due to significant movement of a rock mass. They occur when the stress from rock movements deep below cause the more brittle surface rock to crack or slip. Faults form when stress from underlying rock movements causes the more brittle surface rock to crack or slip. In this area geological forces caused the bed rock to tilt to the south east. Fault lines can be lines of weakness which can then be exploited by natural processes such as water erosion, hence why the stream follows this route.
To the side of the forefinger is a narrow gully. You will notice that the rock looks different from the rock that you are stood on. The rock to the side and forming the narrow gully is known as Fault Rock.
There are different types of Fault Rock:
Fault rock is known as Cataclastic rock. This type of rock has been formed by the continuing fracturing, grinding, and movement of existing rock in a process known as cataclasis.This involves the granulation, and crushing of the original rock.
Cataclasite
This type of fault rock, consists of angular rocks, amongst finer grained material.
Fault breccia
This is a type of fault rock, that consists of large fragments of rocks, amongst finer grained material. It may include mineral veins.
Fault gouge
This is an unsorted type of rock, consisting of finely crushed material. There can be types which have a high clay content.
Pseudotachylite
This fault rock has the appearance of dark glassy rock.
STAGE ONE TASKS
1. To the side of the thumb in the rock bordering the beck, what is carved?
2. How wide is the forefinger?
3. Please describe the colour and feel of the rock which forms the forefinger and thumb.
4. Please describe the fault rock, what type do you think that it is?
TIME TO MOVE ONTO STAGE TWO.
THIS INVOLVES SCRAMBLING DOWN THE NARROW GULLY INTO THE NARROW GORGE.
Please look at the wall of the gorge, (the one which is on the same side as the narrow gully you have just come down) you will notice some lines.These are due to friction, 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.
What we can see here are known as 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. It is possible to work out the direction of the movement along the fault line by looking at what direction the striations point.
STAGE TWO TASKS.
5. Please describe the slickenside, how does it differ from the rest of the rock outcrop? What does it feel like, how does the colour differ from the rock outcrop?
6. On what directional axis did the movement taken place, north / south or east / west?
7. Are the slickensides, vertical, horizontal or angled?
8. Now look at the gorge, how wide is it at this location?
This being an EarthcCche, in order to log it, I ask that you answer the above tasks. 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.