St Ninian's tombolo, the largest geomorphologically active sand tombolo in Britain. A tombolo is a beach or a bar which links two islands or an island to a larger landmass. It is known locally as an ayre, old Norse for 'gravel bank'. The tombolo links the south-west Shetland Mainland to the small off-lying island of St Ninian's Isle. In the Northern Isles, tombolos are generally formed of gravel, cobbles or occasionally boulders. St Ninian's tombolo is distinctive among its fellows in being composed mainly of sand.
An information board can be located at the parking co-ordinates. You will need to answer the following questions to claim this geocache :
1) St Ninian's tombolo is composed almost entirely of medium-grained sand (0.24 mm) with a depth of about 2 metres which overlies a gravel base. What type of sand is the tombolo made from?
2) During low spring tides the tombolo is 60–70 m wide, while during the highest spring tides the central part may be completely covered in water. Typically the central part of the tombolo is 20–30 m wide at high tide. Estimate the length along its central axis.
3) Longshore drift consists of the transportation of sediments (clay, silt and sand) from its source (eroded cliffs) along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. With this in mind describe how the sedimentation came to rest at the area of deposition and form the tombolo.
E-mail your answers to the cache owner. No need to wait for a reply to the answers, but we will delete logs if the answers aren't satisfactory, so you may prefer to wait.
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