To Claim a Find …
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Go to the given co-ordinates, and then go to the given reference point (Second Sink Hole).
There are TWO locations to visit
At each point you’ll find a sink hole.
Once these weren’t the craters they are today. Once the soil in the area was pretty much level.
For each of the two:
Estimate the volume of soil that has gone.
Explain where that soil has gone.
Is the sink hole relatively new or relatively old compared to the other? – justify your answer.
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To estimate the volume of the soil
Assume the sink hole is conical in shape. Estimate the diameter of the hole at its top.
(If it isn’t circular measure the widest and narrowest and take an average to use as the diameter - you won’t be far off)
Take the number you have for the diameter and halve it.
Let this number be “R”
Estimate the depth of the hole. If it is filled with water, make a best guess judging from the angle of the sloping sides.Let the number you have for the depth be “h”
The volume of soil that has gone is
3.14 x R x R x (H/3)
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Please send me your answers either by email via geocaching.com or by the messenger in the geocaching app. Feel free to add photos of yourself or some personal item to your log (but only if you want to), but please be sure not to show anything which might give away the answer – why not take a photo at another sink hole?
To Help You To Answer
Kings Wood sits on the North Downs - a ridge of chalk hills stretching for over a hundred miles from Farnham in Surrey to the White Cliffs of Dover
Chalk is a pure white limestone which is formed from the remains of tiny marine organisms that lived and died in the seas that covered much of Britain around 70 to 100 million years ago. When they died, their bodies fell to the sea bed and compressed together forming the rock we see today. And there’s a lot of flint in with it too too for reasons we explore in another nearby EarthCache.
One thing to bear in mind about chalk is that when soaked with water it can form a semi-liquid slurry.
I regularly walk my dogs round Kings Wood many times each week. It occurred to me that we rarely see much chalk... but there was something about the places where we did see the stuff.
This EarthCache will take you to some of those places.
Sink Holes
A sink hole is a hole in the ground that forms when water acts on near-surface rock.
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Green is topsoil
Tan is the underlying chalk.
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In a landscape where chalk sits underneath the soil, water from rainfall collects in cracks in the chalk.
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Slowly the chalk dissolves… or doesn’t so much dissolve as combines with the rainwater to form a semi-liquid slurry. As the action of the rain carries on working on the chalk so the underground cracks widen and lengthen,
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The dissolved chalk and semi-liquid slurry are washed deeper into the ground and is carried away deeper and deeper leaving empty voids (underground cavern-like openings in the rock)
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The remaining chalk isn’t strong enough to support the weight of the ground above. It becomes unstable and collapses without warning to form sink holes
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Initially the sides of a sink hole will show exposed soil and rocks, but over time vegetation will cover this.Water will continue to drain through the cracks which caused the formation of a sink hole causing further collapses until mud or debris plugs them up, at which point it fills with water to become a lake or a pond. This process may take some time.
The actual size and shape of sink holes depend on exactly what has happened underground. There can be wide but not deep cracks collapsing forming wider and shallower sink holes, or the cracks may form cavern-like hollows in which case the sink holes might not be particularly wide, but can be much deeper. Sink holes can also form when cave roofs collapse. I was going to say that this isn’t the case here in Kings Wood… but who knows what is happening underground?
Sink holes can vary from shallow holes about 1 meter deep to pits more than 50 meters deep. There are a lot of them in Kings Wood, but aren't any quite that big here. I've listed a few that you might like to visit whilst you are in the wood (though visiting these is not a requirement and is entirely optional)
N51 12.835 E000 53.827 - A rather small one as you leave the main car park
N51 12.740 E000 54.534 - A relatively new one fenced off for safety
N51 12.893 E000 54.790 - An old overgrown one on the left of the path
N51 13.024 E000 55.009 - A larger shallow one
N51 13.065 E000 55.076 - A smaller but slightly deeper one
N51 12.856 E000 55.682 - On the North Downs Way
N51 13.383 E000 55.840 - A non-circular one
N51 13.390 E000 54.954 - A rather large older one
There are many more. Depending on whch path you take you will probably pass a sink hole as you go from one of the sink holes I am asking about to the other.
I'll give a special mention to anyone who sends me co-ords of one which is new to me...
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The landowner (Forestry Commission East England) does not assess the suitability or safety of the cache location. The cache placer and the cache finder have a responsibility to take reasonable precautions to protect their own safety and the safety of others. Hazards observed at or on route to the cache location should be reported to the cache owner. The Cache has been placed in accordance with the Forestry Commission East England agreement