Please take note of the terrain rating and be aware you do this at your own risk.
This EarthCache requires you to visit the location given, make some observations, answers some questions and either email or message the answers to me.
Then you can log your find and don’t forget to enjoy the stunning location and views.
At the location you need to be on the stone platform which is just SE of the trig column and is at the top of the rock face.
There is short step over a deep crevasse but the rock platform is about 3m square so quite safe for a few people at once.
Please be aware there is a sheer drop of about 15m so dogs and mini-cachers must be watched carefully.
If you are standing on the rock platform, I wish you to contemplate the surface you are standing on and wonder about the circular hollows, below is some information about how they form and what they are.
When you have had a look and a read please answer these questions.
- please describe the surface of the platform just Se of the trig tower, concentrate on the texture of the surface and any depressions including their shape.
- Please tell me how you think the hollows on the surface have formed
- Please tell me how many of the hollows they are on this rock and the depth of them.
Formation of circular hollows in stone.
The name of the hollows in this type of sandstone is Panhole.
A Panhole is a shallow depression or basin eroded into flat or gently sloping rock, it is not unusual for many panholes to form on the surface of a flat exposed rock.
Panholes are erosional features that are developed in a variety of climatic environments and in a wide range of rock types. These shallow basins, or closed depressions, are quite commonly well developed in surfaces of sandstone. They are generally characterized by flat bottoms and sometimes by overhanging sides. The initial form could be colonised by flora of some type but that is not the case at this location. Diameters are rarely greater than 15 cm (5.9 in). Some panholes were at one time thought to be man-made because their roundness was so perfect they were argued not be natural and must have been shaped by humans.
So how do they form?
They form from a combination of physical and biological weathering and erosion.
Lets clear up the difference between wreathing and erosion first. The primary difference between weathering and erosion is that weathering occurs in place (meaning the rock particles are broken off the main body and yet stay there) whereas erosion involves movement to a new location. Both are caused by similar factors of wind, water, ice, temperature, and even biological action. They can of course also occur together and often do.
So to physical and biological weathering. Physical is caused by the factors mention above, wind, water, ice and temperature, this break off small particles from the rock, making them easy to then be eroded away. However, if this was all that was happening here the whole of the top of the rock would be reasonably flat, surely, not with hollows. Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes. The most obvious would be plant roots that exert stress or pressure on rock causing it to crack and break. Although the process is physical, the pressure is exerted by a biological process and so is called such. Again, this large biological weathering is not the case here because the rock surface has no obvious plants growing on it.
So, what on earth is going on?
Well to answer this question you will have to think small, very small and look very closely at the rock. When you do you might notice that the rock surface is in fact covered with plants, tiny, tiny plants, which are doing the damage and breaking down the rock causing the start of the hollows.
The biological weathering only starts the process however, since once the small formation of a hollow starts the process accelerates, (please remember geological acceleration is still pretty slow), as more water is held and both types of weathering are able to focus on that patch. The retention of water in a small hollow enables the plants to work longer, ice to break the surface and even a little dissolving of the mineral to occur, all of which break off microscopic particles, slowly.
Then the erosion kicks in and the exposed area means the regular winds blow away the particles that have been freed from the rock face. The eddies in the wind will even blow the particles round the bowl a few times before they escape and are blown away, enhancing the shape and deepness of the bowl. The results in a rounder and rounder and deeper and deeper Panhole, fascinating right?
This process takes year, probably millennia to create the deep bowls that can be seen her. Eventually they will join to make bigger and bigger bowls which will then, eventually, result in the whole of the surface being eroded away and the process starts again perhaps. Or there is a maximum depth that they can reach, the depth to which the wind can penetrate and remove the weathered material. Either way the change is slow and we get to witness one of the processes that being out here on the moors afford us.
Well thanks for having a look at this location and this cache.
I hope you found it useful and interesting.
Happy caching.
treboR