A tarn is a mountain lake or pool that occupies a basin excavated by a glacier high in an alpine environment. This body of water is a direct geological signature of past glacial activity in a mountainous region. The lake forms within a steep-sided, amphitheater-shaped depression and is typically filled by accumulated meltwater or by precipitation and runoff after the ice has retreated.
Glacial Origins of Tarns
The formation of a tarn begins with the erosive action of a small glacier, often called a cirque glacier, situated near the head of a mountain valley. This glacier occupies a hollow where snow accumulates and compacts into ice. The ice mass works against the bedrock through two processes: plucking and abrasion. Plucking occurs when meltwater seeps into rock fractures, freezes, and expands, pulling rock fragments away.
Abrasion involves rock debris embedded in the ice grinding against the underlying slope, deepening and widening the hollow. This continuous erosion carves out a characteristic bowl-shaped depression with steep walls on three sides, known as a cirque (or corrie in the United Kingdom). The base of the cirque is scoured into a concave shape suited to holding water.
As the climate warms and the cirque glacier retreats, the excavated basin is exposed. Water from snowmelt, precipitation, and runoff collects in the deep rock hollow. The water is frequently held in place by a ridge of unsorted glacial debris, called a moraine. This moraine was deposited at the lip of the cirque and acts as a natural dam or sill. This sequence of erosion, retreat, and water accumulation defines the geological origin of the tarn.
Defining Physical Characteristics
Tarns are distinguished by their semi-circular or bowl-like shape inherited from the cirque basin. They are generally small in surface area, reflecting their confined origin. Despite their limited surface extent, tarns are often quite deep, with some reaching depths exceeding 30 meters, giving them a high depth-to-surface-area ratio.
Located at high elevations, the water in a tarn is typically very cold year-round and remarkably clear. This clarity is due to the lack of significant sediment input and low biological productivity, classifying them as oligotrophic lakes. Oligotrophic waters are characterized by low nutrient content and sparse aquatic vegetation.
In cases where they are still fed by an active glacier, the water may appear a milky turquoise color from suspended fine particles of rock flour. Rock flour consists of fragments ground to a powder by the glacier’s erosive action.
Geographic Distribution and Terminology
Tarns are found globally in any high-latitude or high-altitude mountain range that has experienced glaciation. They are common features in the North American Rocky Mountains, the European Alps, the Himalayas, and the Southern Alps of New Zealand. Their presence is a definitive marker of a formerly glaciated alpine landscape.
The term “tarn” is derived from the Old Norse word tjörn, meaning a small mountain lake or pond. Its use became localized in Northern England, especially within the Lake District. In other regions, the same feature is known by different names that reflect the local language and history.
For example, in the Scottish Highlands, a tarn is referred to as a “corrie loch,” incorporating the Scottish Gaelic term for the depression. Geologists also use the more general term “cirque lake” internationally. These unique water bodies serve as evidence of the immense power of ice to shape the planet’s mountainous terrain.