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Quintessential Chilterns: Incombe Hole

A cache by Trixie81 Send Message to Owner Message this owner
Hidden : 05/25/2016
2.5 out of 5
2.5 out of 5

Size: Size: other (other)

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Geocache Description:

Magnificent sunsets can be viewed from this spot high on the Chiltern Scarp but how was this feature formed. 

The Chilterns AONB of which Incombe Hole forms part, is perhaps best known for the scarp slope that runs SW -NE north west of London.
The hills are formed by an outcrop of chalk. The chalk strata have tilted to form a gentle dip slope which ends abruptly above the Vale of Aylesbury in the north and the River Thames at the south. The chalk was laid down during the Cretaceous period (about 66 to 145 million years ago) at a time when the entire area was under water.

At some point within that pockets and /or layers of clay and flint were formed. The Chalk seas thrived with life, including sponges and microscopic organisms that built their skeletons from silica dissolved in the sea water. Flint is the end product from a precipitation of silica. On death and burial in the chalk mud, the silica slowly re-dissolved to provide silica-rich water within the sediment. There then follows some complex chemistry in which the silica is precipitated from solution at the boundary of the oxygen-rich sediments immediately beneath the sea floor and the low oxygen sediments below about 10 metres beneath the sea floor. The silica often forms around fossils sponges and the shells of sea urchins (echinoids), which provide a nucleus for the flint to form. However, most flint is formed in ancient burrow systems that provided easier passage for the mineral-rich.
During the Ice Age(s) that began 2.6 million years ago at the start of the Pleistocene epoch, the Ice advanced and retreated from the North and between the various peaks the temperature rose quite considerably to temperatures high than we experience today . This allowed animals such as the hippo, elephant and auroch to roam and survive.
The ice reached its most southern point during the Anglian Ice Age, possibly as far south as Finchley in North London but certainly as far south as the Thames whose course it diverted.The Anglian Ice Age is the name used in the British Isles for a middle Pleistocene glaciation. It precedes the Hoxnian Stage and follows the Cromerian Stage in the British Isles. The Anglian Stage is equivalent to the Elsterian Stage of northern Continental Europe, the Mindel Stage in the Alps and Marine Isotope Stage. The Anglian Stage and Marine Oxygen Isotope Stage started about 478,000 years ago and ended about 424,000 years ago.
The effect of glaciation or if not glaciation, the tundra like conditions that would have occurred (there is some dispute as to whether the Chilterns were covered by glaciers on the highest points) was to expose/lay down in some areas a clay with flint deposit which is significantly different in many ways from the traditional chalk.
Further during the tundra like conditions/glaciation a process known as nivation occurred. This process involves a combination of gelifraction (the fracturing of rock when water freezes in joints or pores) and solifluction (the downhill movement of the debris). These processes are particularly active under tundra conditions when there is an alternation of freezing and thawing either on a daily or seasonal basis. The result is a gradual movement of loose rock debris down slope and small depressions, or combes, formed at the top of slopes. The material that was eroded in the formation of the combe is spread out as broad apron-like fans on the lower slopes of escarpment, but some remains to fill the narrow floors of the combes (geological maps often mark this in as 'Head').

In addition the retreating glaciers (after the Anglian, Wolstonian ( c 250,000 years ago) and Devensian ( 125,000 to 10,000 year ago) Ice Ages cut dry valleys and one result is the rounded hills of the Chilterns, separated by dry valleys. Although these valleys are dry today, they were once the site of torrential flow from the melt waters of a retreating ice sheet and melting snow.
They are common features throughout the Chalk of southern England, not just the Chilterns. The majority show the typical form of a water-cut valley, that is, fairly steep sides, an asymmetry in form and an overall v-shaped profile.
Today water does not flow over the Chalk hills. Chalk is normally a highly porous rock and the numerous fractures and pore spaces ensure water now permeates through the rock very efficiently to the water table. However, under tundra conditions the water in these pores and fractures becomes frozen and any melt-water is forced to flow over the surface. In this way, deep river channels can be cut down relatively
Incombe Hole

From the parking cross the road and walk east north east to N 51 49.718 W 000 36.583.

Please do comply with the advice given and keep to the footpaths. From this point you will walk along the edge of the ‘Hole’

Please send me the answers to the following questions by either email or through the Message Centre:

1. 1 Stand at N 51 49.901 W 000 36.408 and describe the shape of the 'hole' between this point and N 51 49.929 W 000 36.542. Is it a 'V' shape or a 'U ' shape or some other shape?
1.2 Using that information how do you think the 'Hole' was formed ( see description above) .
1.3 Estimate the width of the valley floor.

2. On the opposite side of the Hole you can see a ripple effect- looks like ripples on a pond ( but on the side of a hill!) , what do you think caused this?

3. Describe the course of ‘the Hole’ from N 51 49.929 W000 36.542, what do you think caused it to turn as it does?

4. What is the highest point on the footpath? Using this information , estimate the depth of the Hole.

5. 1 Standing at N51 49.958 W000 36.485 what can you see exposed on the southern face of the Hole?
5.2 What has caused the hummocks in the grass, near where you are standing ?

From this point, please retrace your steps to the parking or continue over the hill towards Ivinghoe Beacon.

Any logs claiming FTF where the finder has not e mailed the correct answers will be deleted.

Additional Hints (No hints available.)

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