The prominent crater row of Lúdentarborgir takes its name from the Lúdent ash ring. It is 15 km long and includes the craters of Grænavatnsbruni which are of the same age, although they are shifted 5 km to the west. The main crater row, which erupted 2300 years ago, extends south from Seljahjallagil to Námafjall in the north. The lava covered most of Lake Mývatn and continued down the valleys of Laxárdalur and Aðaldalur. Most of the lava originates from the southern part of the crater row, and covers 220 km2. The two largest and southernmost craters, called Þrengslaborgir, are located by Bláfjallsfjallgarður. The southern part of the crater row is often called Þrengslaborgir as a whole, but geologists call the lava itself, "Younger Laxá Lava". This lava flow includes Dimmuborgir and the extraordinary Skútustaðagígar rootless cones.