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Hexagons at Giants' Causeway EarthCache

Hidden : 05/27/2015
2.5 out of 5
1 out of 5

Size: Size:   other (other)

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Geocache Description:

An earth Cache located at the bus turn-round spot at Giant’s Causeway

Please visit the location given and message or email the answers to the questions below. 

If you are willing and able please include an amazing photo.

Published with the kind permission of National Trust

Notes about your visit.

This is FREE to enter!

However if you visit the National Trust Centre it will cost you £7.50(booked online in 2015) per-person,(£9 on the day), but if you walk to the location across the cliff path or if you go through the huge arch to the right of the car park and down the bus route (about 1km), it’s free.

This is also wheelchair and pram accessible and if you choose to go on the bus (£1 each way, from the NT visitor centre) it’s even accessible for the less mobile too.

If you do not pay there are still toilets to use but you can’t get into the café.

This is a ‘world heritage site’ an ‘area of special scientific interest’ and a ‘national nature reserve’ so please treat it with the respect it deserves.

Please take away only photos and memories and leave only footprints.


By the way I believe it should be Giants’ causeway not Giant’s because it is the causeway of more than one Giant.




This cache focuses exclusively on the hexagonal columns close to the bus turn around section of the road

Although the Giant’s Causeway is so-named due to an ancient legend, its formation ac-tually began some 60 million years ago when volcanic activity forced tectonic plates to stretch and break, causing magma to spew up from inside the Earth and spill out across the earth’s surface as lava.

The temperature of erupting lava can range from between 700 and 1,200 degrees Celsi-us (1,292 and 2,192 degrees Fahrenheit). However, upon contact with the air it will im-mediately begin to cool. At first this cooling is extremely rapid and causes a hardened crust to form on top of the lava, which insulates the still liquid lava below. Because the lava is insulated this cooling becomes increasingly slow over time. While you could probably walk on the lava’s crust after about half an hour, thick lava flows can take many years to cool completely and become totally solid.

While the temperature falls the lava dries out, and it’s this drying that causes the solidifying lava to crack and form regular pillars of basalt rock. The size and shape of each column is determined by the rate at which the lava cools and dries, and therefore the speed at which what’s called the ‘drying front’ moves. Scientists from the University of Toronto discovered that the slower the cooling the larger the columns, this probably took more than 100 years to cool fully.


We tend to think of this formation of rock to be unique but here are the locations of another 10 similar locations around the world


Fingal's Cave on the Isle of Staffa in Scotland


Devils Postpile National Monument in California


Hexagon Pool in Israel


Svartifoss Waterfall in Iceland


Takachiho Gorge in Japan


Los Prismas Basálticos in Mexico


Cape Stolbchaty in Russia


 Los Organos of the Canary Islands in Spain


Ghenh Da Dia in Vietnam 


Basalt Cliffs of Jeju Island in South Korea


You can find more information and photos of these sites at


Gabbro and Basalt


What is Gabbro?


Gabbro is a coarse-grained, dark-colored, intrusive igneous rock. It is usually black or dark green in color and composed mainly of the minerals plagioclase and augite. It is the most abundant rock in the deep oceanic crust. Gabbro has a variety of uses in the construction industry. It is used for everything from crushed stone base materials at construction sites to polished stone counter tops and floor tiles


What is Basalt?


Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is a coarse-grained rock.


Gabbros are equivalent in composition to basalts. The difference between the two rock types is their grain size. Basalts are extrusive igneous rocks that cool quickly and have fine-grained crystals. Gabbros are intrusive igneous rocks that cool slowly and have coarse-grained crystals


It is often stated that Earth's oceanic crust is made-up of basalt. The word "basalt" is used because the rocks of the oceanic crust have a "basaltic" composition. However; only a thin surface veneer of oceanic crust is basalt. The deeper rocks of the oceanic crust are generally coarser-grained gabbro. Basalt occurs at the surface of the crust because the rocks there have cooled quickly, (in a geological timescale). At greater depth the cooling rate is slower and large crystals have time to develop.




As you look at the columns you will notice that some are concave like a seat and some are convex like a pillow this is called horizontal corrugation. The horizontal corrugation or striations on the vertical column surfaces are the result of the stepwise downward advance of the fracture pattern. This means if adjacent columns are unequal in size, the asymmetry of the cooling between the two columns will drive the crack toward the larger column. Hence, the smaller column will form corrugations with convex surfaces, and the facing surface of the larger adjacent column will form concave surfaces. So if you look you should find convex and concave columns next to each other and the smaller of the columns should be the one you feel inclined to sit on.


Ok so now you should be able to answer the questions below.

  1. 1) Please describe the stones, I know they are mostly hexagonal, be creative, particularly concentrate on the grain or crystal size
  2. 2) Tell me if you think these rocks are basalt or gabbros? (Hint geologically 100 years is the blink of an eye, slowly means 1000 years or more)
  3. 3) Some columns are concave, (seat) some convex (pillow). Which tend to have a larger cross section, concave or convex?
  4. .

    Finally, if you are willing and able please include a photo in your log.  It is almost impossible to take an un-amazing (is that a real word?) photo around here so don’t worry about giving away answers.

    Thanks for visiting the location;

Additional Hints (Decrypt)

Gur thvqrf ner urycshy

Decryption Key


(letter above equals below, and vice versa)