BECKYHITCHIN
As above , so below
Becky Hitchin takes a deep dive into the geology of the United Kingdom and the world that awaits below the water line . Part 1 : Scotland
There are many reasons I love shore diving . One of them is that often you can look at the land around the dive site and know what you ’ re expecting to discover underwater . When rocks plunge from the shore into the sea , you know that they are likely to continue as underwater walls , at least for a little while .
When you walk in from a sandy beach , again you ’ re generally expecting that habitat to dominate the dive . And why is that ? It ’ s because the bottoms of our shallow seas are the result of millions and millions of years of tangled and twisted geology and sedimentology . And I ’ m going to take you on a tour round the UK and see what we can discover about how geology influences our diving .
I ’ m going to start with West Scotland . Yes , I ’ m biased . But it ’ s also a destination loved by many UK divers . West Scotland is made from ancient rock , most recently shaped by constant movements of ice in the glacial periods of the Quaternary – erosion , deposition , erosion , deposition repeated for thousands of years . This left a complicated seafloor pattern of some basins that were deeply eroded and some that became filled with deep layers of sediment .
The Scottish sealochs , basins often reaching hundreds of metres in depth and separated from the open sea by shallow sills and tidal narrows , are part of the result of these glacial processes . They are home to deep muddy habitats that contain species that would usually be found in much deeper water , such as tall seapens , firework anemones and spotted dragonets .
As you all know , Scottish water is always cold compared to water ‘ down south ’, with the temperature varying only slightly through the year , from
“ These huge Atlantic water masses help provide the incredible visibility ”
about 9 ° C to 13 ° C at the peak of summer . So … why is it so cold ? There are several reasons . In winter and spring – well , and often in summer and autumn too - there ’ s snow on the hills , and freshwater runoff tumbles down gin-clear rivers to the shore , causing a dark , clear , layer of freezing cold , peaty freshwater to settle on top of the denser seawater already present .
Safety stops become somewhat nightmarish when you ascend into this layer and have to wait in 3 º C water after a long dive . But it ’ s also cold from the seaward side of things too . Water masses tend to warm up near to the shore if there are shallow areas for the sun to heat , but there ’ s very little shallow water to warm up around Scotland . Scotland ’ s waters are strongly influenced by Atlantic Ocean water masses , mixing into deep shoreward waters . These huge Atlantic water masses help provide the incredible visibility you see when diving at St Kilda , the northern Scotland islands and even Kinlochbervie .
The glacial landscape continues in Scottish seas , giving rise to complex , twisted topographies , often plummeting into the depths , and longshore currents lead to many of those rock surfaces being clean of sediment but full of life . Even from the shore , the underwater landscape can contain dramatic walls and slopes , such as the Kentallen Wall that plunges without warning from a 10-15m sandy slope to a vertical wall leading to 40m plus . Further out , think of places like Bo Fascadale , Maxwell Bank , Handa Island . It ’ s an incredible experience stopping in blue water , no land for miles , and rolling back into the water to look for the top of a pinnacle 6m or so below you .
This landscape continues , getting wilder , as you round the top of Scotland , eventually with water , wind and waves funnelling themselves into the Pentland Firth between Orkney and the mainland . The Pentland Firth wasn ’ t swum across until 2011 , and with tidal flows reaching 12 knots , it ’ s maybe not surprising .
Beyond the Pentland Firth , the land bends round on itself to form the north east coast of Scotland , passing the famous John O ’ Groats and forming a sweep of cliffs down to Inverness and the next obtuse angle heading into the Moray Firth and the North Sea . This is a very different matter to the oceanic swells caressing into the west coast . Only the very top of the North Sea has any immediate Atlantic inflow , instead being dominated by recirculating nearshore currents around the North Sea basin , from Scotland to England , across to continental Europe and back up towards Scandinavia .
The sea shallows as you head down Scotland , waves breaking along high cliffs where gannets and fulmars roost and cry . By the time we reach St Abbs and Eyemouth , the best-known dive sites in the east of Scotland , even the rocks have softened . One of the main reasons that we have such good dive sites there is that St Abbs Head is volcanic , leaving steep sided gullies and sea stacks above and below water as it erodes , compared to the softer sedimentary rocks on each side of it . The water continues moving south , enough to sweep away sediment and provide constant food for the abundant life seen underwater . St Abbs and Eyemouth are also placed a good distance away from major river mouths , particularly the Tay , the Forth and the Tees , which makes the amount of sediment found there small compared to dive sites further north and south . There ’ s also Arctic influence , with species more commonly seen in northern waters being found in abundance at St Abbs – especially wolf fish [ see Lawson Wood ’ s feature in last month ’ s SCUBA ].
As we head into England , I ’ m leaving you there until next month . And yes , sorry to my friends and colleagues in Orkney and Shetland , you have been unloved and ignored in this column . Don ’ t worry , your time will come … �
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