Welcome to Northmavine - Mavis Grind EarthCache
Welcome to Northmavine - Mavis Grind
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This Earthcache concentrates on the geology of the most northerly section of Shetland Mainland - Northmavine. It is located at Mavis Grind, the narrow strip of land (isthmus) connecting Northmavine to the rest of the Shetland mainland.
Shetland has a fascinating geological history which begins many millions of years ago. Northmavine boasts some of Shetland's best geodiversity and some of the oldest rocks in the UK. Approximately 20 different rocks are evident in this area, ranging in age from 350 million to 2.2 billion years, the oldest of which are around half the age of the Earth itself.
The key stages in Shetland's geological development are briefly outlined below:
600 million years ago the Earth and the continents did not look as they do today. Shetland and Scotland were part of an ancient (paleo) continent called Laurentia, most of which later became North America and Greenland. Laurentia was located near the South Pole. The seabed around it was covered in thick layers of sand and mud, which turned to sedimentary rock over time. The Iapetus Ocean separated Laurentia from Avalonia (northern Europe) and Baltica (western Europe). See diagram.
The tectonic plates which carry the Earth's continents are constantly shifting, and around 500 to 420 million years ago Laurentia, Avalonia and Baltica collided, forming a huge new landmass called Pangea. The sedimentary rocks around Laurentia were crumpled and forced upwards, creating the huge Caledonian mountain chain, which would have been as large as the Himalayas today. At a similar time, parts of the Iapetus seabed were also forced upwards and onto the continental rock. Exposed seabed rock like this is called Ophiolite. The Serpentine rocks on Unst and Fetlar are examples of Ophiolite.
Approximately 400 million years ago, Shetland had shifted so far north that it was located close to the Equator. During this time, it had a tropical climate, with a combination of desert and hot and humid areas. The area around Lerwick would have been similar to California's Death Valley... very different to today!
Continental collision generates a lot of heat. Around 360 million years ago this caused rocks to become molten deep below the surface. The resulting magma forced itself to the surface creating volcanoes. One of these was located in the Eshaness area of Shetland.
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This diagram shows the Earth as Pangaea was rifting apart approximately 175 million years ago. The new continents shown below are Laurasia to the north and Gondwana to the south.
The North Sea formed between the UK and mainland Europe around 300 million years ago. Another larger rift created the Atlantic about 60 million years ago, finally splitting Europe from North America.
The geological story does not end here. The continents continue to move, with North America and Europe still drifting apart by a few millimetres each year.
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As a result of the processes above, all three major rock types are present in Northmavine. These are:
igneous: volcanic rock which originated as lava or magma
sedimentary: which was created from compressed sand and mud
metamorphic: which are rocks that were changed under immense heat and pressure
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This Earthcache makes use of The Wall (or "Da Waa" in Shetland dialect), which is one of many geological exhibits across Shetland. It was constructed at Mavis Grind in 2007 to demonstrate the complex geological structure of the Northmavine area. It is a large cross-sectional model and was created as part of the Shetland Geopark project, led by the Shetland Amenity Trust. The rocks in The Wall were collected from actual locations across Northmavine, including the top of Ronas Hill, by Tom Jamieson and Adam Brown.
In September 2009, Shetland was accepted into the European and Global Geoparks Network. A Geopark is an area with outstanding geological heritage, and which using that heritage to provide sustainable economic benefits to the area. Shetland Geopark is one of 66 global geoparks, supported by UNESCO. Details about the Shetland Geopark can be found at www.geoparkshetland.org.uk
For further information about Geoparks in general, see www.europeangeoparks.org
To claim this Earthcache, you will need to use the resources at the co-ordinates given.
a) What is the name of the rock photographed above? Is it igneous, metamorphic or sedimentary? What do you think has caused the holes in it?
b) Why are so many of the rocks in Northmavine of igneous origin?
c) Locate the layer of “crushed and finely powdered rock” on The Wall. Which feature does this border, and why do you think the rock has been damaged in this way?
Please send the answers to me (buffhusky) using Geocaching.com. Please do not post answers in your log as I will unfortunately have to delete them to avoid spoiling the cache for others. Photos will be very welcome!
Congratulations to cumlietwo for a very speedy FTF!
Of further interest: There is also an interpretation board at Mavis Grind which explains the botany, archaeology and tank traps in the area. It also illustrates how the isthmus was used as a crossing point for boats to save a long journey around a hazardous coastline. If you are very lucky, you may see otters use it for the same purpose today!
Additional Hints
(No hints available.)