The Younger Laxá lava was formed in Lúdentarborgir about 2300 years ago. It flowed over Lake Mývatn, and sank into the muddy lake bed where the boiling water caused it to be shredded into scoria and spatter. Rootless cones built up around the eruptive openings, while molten lava feeding remained and irregular crater clusters such as Skútustaðagígar formed in and around the lake to the south and west. Amidst the scoria one can find reddish baked diatomite. Another variation of rootless cones appear in Dimmuborgir, where a threshold of scoria had dammed a reservoir, into which lava then flowed, forming chimneys which spewed out steam and tephra. Gradually a low lava bulge formed and the lava pond got crusted over. When the molten lava drained away the solid crust came down leaving scratch marks behind on the inner walls of the pond and outer walls of the chimneys. In places caves have formed where drainage channels emptied.