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Greenwich Foot Tunnel Dimension Stone EarthCache

Hidden : 02/20/2017
2 out of 5
1 out of 5

Size: Size:   other (other)

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Geocache Description:

The Royal Borough of Greenwich have given permission for this EarthCache to be sited here. This EarthCache is located within an area frequently patrolled by Police. Please avoid acting suspiciously whilst completing the Tasks, and, if challenged, please freely explain about geocaching. It may be worth pointing out that it is not a physical box you are looking for, but are simply studying the rocks visible to everyone, and the person who set the Tasks DOES have permission to do so.

Take a walk under the River Thames to discover a special type of stone that is used in this foot tunnel that connects the north and south of the River Thames.

There are three types of rock:

  • Igneous; they form from the cooling of magma or molten rock. The crystal size is determined by how quickly the rock cools – rocks that cool more slowly below the surface of the earth have a larger crystal size, while rock which cools quickly at the Earth’s surface have a glassy structure.
  • Metamorphic; they are formed through the change (metamorphosis) of igneous, sedimentary, or pre-existing metamorphic rocks. The change is caused through extreme pressure and / or heat which chemically changes the structure of the rock. They can form both underground and at the surface.
  • Sedimentary; they are formed when sediment solidifies and hardens. They can be created from organic remains (such as shells), or form from the cementing of other rocks. The material from which sedimentary rocks are formed affects the structure and grain size.

Examples of all of these types of rock can be formed into 'dimension stone'. Dimension stone describes rock that has been cut into specific sized and shaped slabs or blocks for building purposes. The shapes and sizes of dimension stone vary enormously, as does the types of rock used, dependent on purpose. Dimension stone has been used in building for millennia, with rocks selected for their specific properties in terms of durability, strength and the finish required. We will examine dimension stones formed from one type of igneous, metamorphic and sedimentary rocks.

IGNEOUS: Granite

Granite is a light-coloured igneous rock with crystals large enough to be visible to the naked eye. It forms when magma crystalizes below the Earth's surface. The large crystal size provides evidence that the cooling process was slow. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. The mineral composition determines the colour of the rock, with the most common colours being grey, pink or white, with other colours visible in the grains. Granite is the rock most often quarried as a dimension stone due to its properties as a building material. Granite is hard, meaning that it can resist abrasion, strong so it can bear significant weight, relatively inert which allows it to resist chemical weathering, and it can be highly polished. These characteristics make it a very desirable and useful dimension stone. Granite is commonly used for counter tops, floor tiles, paving stone, curbing, stair treads, building veneer, and cemetery monuments.


Marble is a metamorphic rock that forms when limestone is subjected to high pressure and temperature. This is because the calcite from which limestone is formed changes composition under these conditions and recrystallizes forming a denser structure of more equally proportioned interlocking calcite crystals. It may also contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite, which influence the colour. Crystals in marble are large enough to be visible to the naked eye. Marble occurs in large deposits that can be hundreds of feet thick and geographically extensive. This allows it to be economically mined on a large scale as a dimension stone. Marble can be cut, split or sawed into a wide range of shapes and sized blocks and slabs which can achieve a highly polished finish. This makes it suitable for use in monuments, buildings, sculptures, paving and smaller scale deployment such as work tops or tiles.


Sandstone is a sedimentary rock formed as sand grains or sediment is deposited layer by layer in a body of water. The particles of sand usually range in size from 0.05 millimetres to 2 millimetres in diameter and are tightly grained. Sometimes the layers of sand are separated by thin layers of silvery mica flakes which settle when the water is still. In some sandstones, ripple-marks can be seen running across the surface.  These are formed when the thin layers of sand grains are ruffled by the currents. Sandstone can be quarried as a dimension stone. The most common is flagstone - a shallow naturally irregular-edged slab of stone, sometimes sawed into a rectangular shape.  This is particularly suited to paving, as when sandstone is subjected to wear and tear, the non-slip surface is renewed. It can also be split into very thin layers, which are suitable for use as roofing tiles. In the past, sandstone was used in monuments, but due to its vulnerability to chemical forms of weathering (such as acid rain), it is rarely used in this application now.


Your task to complete this EarthCache is:

Descend into the tunnel, either by the 100 steps on the south side (or 87 steps on the north side), or the lift at either end. As you reach the tunnel, look down at the floor.

1.       Describe the rock – colour, crystal or grain size, texture/finish and general appearance. You can do this at any point within the tunnel, except in the short narrower section damaged by bombs in WWII, bound by steel rings on the north side of the tunnel.

2.       Which of the rock types described above do you think has been used to form the floor? Why?

3.       What makes this type of rock well suited to this purpose and location?

4.       The rock has been chosen for its properties as a dimension stone. Estimate the length and width of each slab (they vary slightly in width, but choose the average size).

Optional: Take a photo of yourself or GPSr in the tunnel. Please try not to give away answers to the questions above in your photo.

Please send your answers to me via email or the Message Center.

Greenwich Foot Tunnel is a heavily used facility in a busy location – it sees more than one million journeys a year by pedestrians and cyclists. Please be considerate to other users and do not cause an obstruction in the tunnel. If possible, time your visit at a quieter time (it is particularly busy in the ‘rush hour’, i.e. 07:00-08:45 and 16:00-18.00).

Please note, the tunnel is served by a lift each end which makes it wheelchair accessible, however these are sometimes out of service. If this is the case, the cache is not wheelchair accessible. You can find out about scheduled maintenance here:

Additional Hints (Decrypt)

Ybbx qbja. Na nqqvgvbany uvag pna or sbhaq ba gur vasbezngvba obneq nobhg gur sbbg ghaary bhgfvqr gur ragenapr ba gur fbhgu fvqr.

Decryption Key


(letter above equals below, and vice versa)