What geological feature is described as a long, deep, narrow lake formed at the bottom of a U-shaped valley?

A long, deep, narrow lake formed at the bottom of a U-shaped valley is specifically identified as a paternoster lake. This feature typically occurs as a result of glacial activity, where glaciers carve out U-shaped valleys and sometimes create a series of small lakes, often connected by streams. The term "paternoster" is derived from the resemblance of these lakes to a rosary, with the lakes appearing like beads strung together. This formation usually occurs in regions that have experienced glaciation, where the presence of the glacier leads to the distinctive shape of the valley and the formation of these lakes in depressions left behind.

In contrast, glacial lakes, while a broader category that includes lakes formed by glaciers, do not specifically refer to the sequence and shape of paternoster lakes. Rift lakes are formed by tectonic activity and are typically much larger and more irregular than the narrow, elongated form associated with paternoster lakes. Lagoons are coastal features formed by the action of waves and tides, typically separated from the ocean by a barrier, which is fundamentally different from the glacially carved features in question.