The largest active shell sand tombolo in the UK, that tells the story of Shetland’s fate following the retreat of the last ice sheets
There can be few places on the planet as beautiful as St Ninian’s Isle on a fine day. The island is connected to the Shetland mainland by a 500-metre stretch of glorious golden shell sand, perfect in its symmetry. Additional interest includes areas of dunes and hill machair adjacent to the tombolo at both ends.
St Ninian’s Tombolo formed as the result of ocean-swell waves from the west diffracting and refracting around the island then converging on the lee side. As sea level rose during the Holocene, quantities of sand were transferred shorewards from the sea floor, forming the sweeping arcuate beaches back-to-back as the waves refracted in the shallower water closer to the land.
The tombolo is rarely covered, although this can occur during spring tides and major storms. However, it does change its shape on a seasonal basis, tending to be narrower and lower in winter when subject to destructive wave action. Its relative stability may be partly the result of a shingle core underlying the sand.
The tombolo is one of many geomorphological features in Shetland that illustrate the story of a recently drowned landscape. Shetland has been progressively submerging since the ice caps melted towards the end of the last glaciation, about 15,000 years ago. Vast quantities of melting ice resulted in rising sea levels globally, but that is only part of the story. Mainland Scotland was covered by a huge ice cap of such weight that the land was actually depressed beneath it. As the ice melted, the land released of the weight began to rebound – a process known as glacio-isostatic uplift. So far, mainland Scotland has been rising faster than the seas around it. This is sadly not the case for Shetland, which had a much smaller and thinner ice cap. As the ice melted, Shetland did not rebound to the same extent, so that the rising sea level outpaced the local uplift, resulting in the progressive submergence of the land. If the water was drained away, we would be left with a landscape of hills and valleys. Islands such as St Ninian’s Isle are simply the tops of hills poking defiantly up out of the waves.
Fortunately, the island should not be fully submerged for a long time yet, and is worth a visit as the place where a hoard of Pictish treasure dating from the second half of the 8th century was discovered under a sandstone slab on the floor of an abandoned and buried medieval church during archaeological excavations in 1958. The treasure, including decorated silver and silver-gilt brooches and bowls, is now on display in the National Museum of Scotland in Edinburgh.
Text contributed by Robina Barton and John Gordon
Find out more
Geopark Shetland, a UNESCO Global Geopark: https://www.shetlandamenity.org/geopark-shetland
Flinn, D. 1997. The role of wave diffraction in the formation of St. Ninian’s Ayre (Tombolo) in Shetland, Scotland. Journal of Coastal Research, 13, 202-208.
Flinn, D., 2003. The origins of St Ninian’s Ayre (Tombolo), Dunrossness, Shetland. The Shetland Naturalist, 2(1): 15-27.
Hall, A. and Fraser, A. 2016. Shetland Landscapes.
Hansom, J.D. (2003). St Ninian’s Tombolo, Shetland. In: May, V.J. & Hansom, J.D. Coastal Geomorphology of Great Britain. Geological Conservation Review Series No. 28. Joint Nature Conservation Committee, Peterborough, pp. 458-462.
This web page is published by the Scottish Geodiversity Forum under a Creative Commons ‘Attribution Non-commercial’ (CC BY-NC) licence, which permits non-commercial reuse provided the original work is properly cited.