Long hyaloclastite ridges are typical features in the area, and unique to Iceland. The ridges are most prominent between the Tungnaá and Skaftá rivers, e.g. Grænifjallarður and Fögrufjöll. The hyaloclastite ridges formed in eruptions below Ice Age glaciers. Lake Langisjór is a long mountain lake between two ridges at the margin of Vatnajökull glacier. The lake is 20 km long, up to 2 km wide and has been measured 73.5 m deep. Farmers from the lowland region discovered Langisjór in 1878 while searching for sheep.