GC49AR6 Great Gibraltar Sand Dune (Earthcache) in Gibraltar created by Gibmetal77

GC49AR6 ▼

A cache by Gibmetal77 Message this owner

Hidden : 4/6/2013

Difficulty:

Terrain:

Size: Size: other (other)

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Geocache Description:

Get to know the ancient Great Gibraltar Sand Dune by learning about the geoscience behind these once shifting sands and how it served the population of Gibraltar for the best part of the last century.

What is a dune?

In physical geography, a dune is a hill of sand built up by either wind or water flow and can occur in different shapes and sizes. Most dunes are longer on the windward side where the sand is pushed up the dune creating a gentler slope and have a shorter "slip face" in the lee of the wind creating a steeper slope. The valleys or troughs left between dunes are called a slacks. An area covered by extensive sand dunes is called a "dune field".

It is not uncommon to find coastal areas with one or more sets of dunes running parallel to the shoreline directly inland from the beach. The dunes play a vital role as natural sea defences as they protect the land against potential ravages from the sea by storm waves. Although the most widely distributed dunes are those associated with coastal regions, the largest dune fields are found inland in the world’s most arid regions and associated with long gone ancient lake or sea beds.

Dune types

Sub-aqueous dunes

Sub-aqueous (underwater) dunes (also known in geology as megaripples) form on a bed of sand or gravel under the actions of water flow. These are very common in natural channels such as rivers and estuaries, and also form in artificial canals and pipelines. Dunes move downstream as the upstream slope is eroded by water flow and sediment deposited on the downstream or lee slope in typical bedform formation.

Lithified dunes

A lithified (consolidated) sand dune is a type of sandstone that is formed when a marine or windblown sand dune becomes compacted and hardened over time. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the colour of the sandstone. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in the Zion National Park in the western United States of America.

Coastal dunes

Dunes form where constructive waves encourage the accumulation of sand and prevailing onshore winds blow this sand inland. Obstacles such as vegetation, pebbles and/or rocks trap the moving sand grains. The sand grains then begin to accumulate, marking the inception of dune formation. The wind then starts to affect the mound of sand by eroding sand particles from the windward side and depositing them on the leeward side. This gradual action causes the dune to “migrate” inland. As the dune shifts, it accumulates more and more sand. Dunes provide privacy and shelter from the wind.

Great Gibraltar Sand Dune

The Great Gibraltar Sand Dune is an ancient coastal sand dune in the British Overseas Territory of Gibraltar. It forms part of the Upper Rock Nature Reserve and dominates the eastern cliffs of the Rock of Gibraltar. It was once used to capture rainwater which was stored in underground tanks within the Rock to satisfy the population's potable water needs.

The windblown sand came from the ancient savanna which once extended to the east of the Great Dune tens of thousands of years ago when Neanderthals roamed Gibraltar.
It’s shape is mostly linear and its lee side is not visible as the dune was blown right up against the Rock’s near vertical eastern cliffs. The sands of the Great Sand Dune have become lithified over the millennia meaning that these sands are no longer shifting.

How to log this EarthCache

To log this EarthCache as a find, you will need to visit the Great Dune to answer the following questions correctly, sending your replies to me for verification before you log it:

1) Describe the sand visible from GZ (colour, grade, etc.);
2) Describe the cross-section of the lithified dune visible from the question waypoint;
3) Of what geological epoch (age) is the Great Gibraltar Sand Dune?
4) What is the name given to the eastern wind which formed the Great Gibraltar Sand Dune and is still the prevalent wind in the area of the Strait of Gibraltar?
5) What was the maximum area of the Great Gibraltar Sand Dune that was once covered by water catchments? (CLUE: You will not find the answer to question 5 at the site)
There is no requirement to submit a photo but it would be nice if you could share one of yourself with the dune in the background in your log... But please no spoilers!
I hope you enjoy and learn from this EarthCache... Happy caching!

All answers sent via message are read and checked but you will not necessarily receive a reply confirming you may log as found. However, please note that incorrect and/or incomplete answers may lead to logs being deleted without notice.

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