The co-ords will take you to a cobblestone wall. Hundreds of people travel past this everyday, and take no notice of it, however once you stop and take a closer look it reveals fascinating geological and local history.
Rhomb porphyry is a volcanic rock with large porphyritic (a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group) rhomb (diamond) shaped phenocrysts (this is a term used to describe large crystals) embedded in a very fine-grained red-brown matrix.
Rhomb porphyry lavas are only known from three areas: the East African Rift (including Mount Kilimanjaro), Mount Erebus near the Ross Sea in Antarctica, and the Oslo graben in Norway.
Porphyry rocks are formed when a column of rising magma is cooled in two stages. In the first, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains.
In this case the rocks could not have ended up here by natural means, so how did they get here? A clue can be found close by, the name of the nearby wharf, Baltic Wharf. In Victorian times an Englishman living in Helleslund, Norway, established a shipping trade between there and here at Mistley. The cobblestones were used as ballast in the ships, many tons of these stones would have made the journey across the North Sea, some were used in buildings or walls, others would just been dumped. Locally this wall has been named 'A Museum of Rare Stones'.
To claim a find on this earthcache please send me the answers to the following questions:
1) Have a look for the red/brown stones in the wall, there are quite a few once you realise which ones they are, these are rhomb porphyry. Observe the crystals in them, and describe the colour to me.
2) Estimate the length of the largest crystals. Explain to me how they were formed, and would they have been cooled quickly or slowly?
3) Describe the texture of the stone and crystals to me, some examples could be rough, smooth, jagged, rounded etc.
4) It would be awesome if you could post a photo of you standing by the wall, but this is optional.