When over 1200 swimmers enter the water for the Gaelforce Great Fjord Swim, most will probably be thinking about the cold, their pace, the conditions and how far they still have to go. Some will be thinking about sharks.
Few will be thinking about the landscape underneath them.
Yet beneath the surface of Killary Fjord is the continuation of one of the most remarkable landscapes in Ireland: an ancient mountain valley, carved deeply by ice over 15,000 years ago then drowned by the Atlantic Ocean.
Killary marks the border between Galway and Mayo, surrounded by some of the most dramatic mountain scenery in the west of Ireland. Glaciers moving through this landscape eroded a broad, steep-sided U-shaped valley. When the ice retreated and sea levels rose, Atlantic water flooded the valley.
That drowned glacial valley became Killary, Irelands only true fjord.
You can still read the work of the ice throughout the surrounding landscape. Corries (semi-circular hollows on the cliffs), sharp arêtes (knife edge ridges), hanging valleys and many smaller signs of glacial erosion are visible all around Killary. Underneath all of this is much older limestone, a 450 million year old geological foundation formed well before the glaciers arrived.
The mountains don't stop at the shoreline, and this is perhaps the part we find most interesting. When you stand beside Killary, your eye naturally follows the mountains down towards the water and stops at the shoreline.
But the landscape doesn't stop there at all, the valley simply continues beneath the sea.
Killary extends roughly 15 kilometres inland. Its water averages around 15 metres deep, but depths towards the outer harbour reach approximately 45 metres.
So while a swimmer sees a relatively calm sheet of water below their goggles, beneath them is a submerged glacial valley with its own slopes, channels and rocks and ridges.
The long, narrow shape of Killary Fjord also affects what happens in the water too, there are more layers to this place that you cant even see.
Salty Atlantic seawater enters from the west, while fresh water flowsoff the land from rivers all along, including the Erriff at its eastern end. Depending on rainfall, tides and wind, the water can become stratified, with a layer of fresh water several metres deep floating on the salty water below.
So a swimmer crossing Killary isn't just travelling over an unusual landscape. There can be an entirely different structure to the water underneath them too. This also makes swimming the fjord unpredictable, if there is a fresh water layer present, the swimmer has much less bouyancy, and has to work harder to cover the distance.
These are just a few of the reasons Killary was such an interesting landscape for us to map.
Our collaboration with Gaelforce will see winners across 16 categories at this year's Great Fjord Swim take home a handcrafted 3D Outcrop map of Killary, individually engraved with their event and winning category.
On one level, each map is a record of an achievement, but it is also a record of the place where that achievement happened. The mountains, the glacial valley, the Atlantic water and the hidden topography beneath it are all part of the same landscape.
So when this year's swimmers look down into Killary, there is rather more underneath them than dark Atlantic water.
