A mountain pool or lake formed in the cirque of a glacier is known as a tarn, a rock-basin lake, or a corrie loch. A tarn is created when either river or rainwater fills up a cirque. The depressions that have been carved out from weak rocks are then occupied by a series of rock-basin lakes. Just like a sponge, the bedrock seeps water, and when that happens, it fills the cracks in the bedrock and the water freezes thus causing the fractures to grow wider. As a result, a moraine may form a natural dam below a corrie loch. If a tarn is still associated with moving glaciers, it will most likely be filled with tiny glacially-ground sediment that scatters light and makes the water appear colorful. Therefore, the color of a glacier’s tarn is the best way to know whether or not the ice is still actively moving. Similarly, crevasses on the surface of the glacier are another indicator that the glacier might still be active in motion.
Tarn is a term derived from tjörn an Old Norse word meaning "pond." The term’s more specific use as a mountain lake comes from the upland regions of Northern England where tarn is the name given to all ponds. The term retains a broader use since it may refer to either any pond or small lake regardless of where it is located or its origin, a good example is the Talkin Tarn found near Brampton in Cumbria, England. In the Scandinavian languages the terms tjørn, tärn, tjern, or tjärn is used to refer to a small natural lake that is often found closely surrounded with vegetation, growing in a tarn or a forest.
Cirque is a French word meaning circle, but in geology, a tarn is an amphitheater-shaped basin that has walls that are steep on the glacial valley’s head. A cirque forms as a result of soil erosion from beneath a glacier’s bergschrund which happens to be an enormous crevasse lying near the rock that is exposed thus separating the moving ice from the stationary ice. During early summer the crevasse opens up and exposes the rock to diurnal temperature changes at its bottom. The lower rock then rapidly disintegrates due to frost action causing an avalanche of the rock at the uppers end, and in turn a headwall that is almost vertical is created. The resulting rock sediments are lodged and carried with the glacier scouring a floor to form concave which may have a tarn if the glacier finally disappears. The expansion of cirques that border each other creates sharp horns, aretes, and cols. Since glaciers originate from the snowline or above it, the study of ancient cirques and their elevations offers useful information on the snow line’s former position as well as climate change.
Examples of Rock-basin Lakes
Lake Ellen Wilson is an excellent example of a rock-basin lake, as well as the Verdi Lake found in the Ruby Mountains of the US state of Nevada. Slovakia’s largest and deepest rock-basin lake is known as Veľké Hincovo. Other examples of tarns include Lake Tear of the Clouds which is in the Adirondack Mountains found in the US state of New York, also the Lousy Lake located in the North Cascades National Park, Picket Range in Washington.