How are tarns made?

Dotted around the mountains of the UK and the rest of the world are the secluded bowls of mountain lakes, more commonly known as tarns. They are sheltered and high, providing habitats for animals and plants and enriching the world of mountain walkers, spectacularly offsetting the rocky peaks above.

In mountains that have been subject to glaciation, many tarns are found in the basins of the great glacially gouged bowls known as cirques, which are also called cwms in Wales and corries in Scotland. The action of glaciers in forming these cirques explains the presence of the tarns within them, and understanding this history can give you an insight into the processes that have been going on in the mountains for millions of years.

Ice ages start with the slow accumulation of snow and ice year on year, which fails to completely melt in the summer and which eventually grows to a point where gravity sends it downhill, forming a glacier. The places where these snow accumulations begin are the high sheltered parts of mountains, where snow is deposited on the lee slopes away from the scouring effect of the wind. Over consecutive ice ages the great cirques form from continued erosion by ice, leading to the creation of steep crags and knife-edge ridges.

But as the glaciers that used to inhabit them moved downhill, what leads to the development of depressions big enough to contain standing bodies of water? There are actually two answers to this, and on closer inspection of any particular tarn, you can see which method produced it.

For the first method, imagine a cirque filled with ice. As it's the highest part of a glacier, it's the first to form and the last to melt. Due to weight within the ice, this great 'semi-sphere' of ice actually rotates in the cirque, eroding the bottom and forming a hollow. Long after the main glacier is gone, this remaining ice continues to gouge downwards, and when it eventually melts, there is a depression big enough for a tarn to fill it, with a rock wall on the downhill side preventing it from draining.

For the second method, imagine again a cirque filled with ice after the main glacier flowing away has all melted. On the open side of the cirque, rockfalls can occur after the pressure of the ice has gone, forming a moraine which acts as the fourth side of the depression, which prevents water from leaving and leads to the formation of a tarn.

There are mountain lakes that form for other reasons, but if you consider the high tarns of Snowdonia, the Lake District and Scotland, you will see they are nearly all surrounded on three sides by rock walls in the shape of a three-sided bowl (the cirque), and on the other side by the drop-off to a valley. For text-book examples go to see Red Tarn on the eastern side of Helvellyn and Llyn Idwal on the eastern side of the Glyderau. Next time you're out why not try and spot one of these classic glacial features!