A U-shaped depression about 3.5 km long, surrounded by cliffs up to 90 m high with a huge crag (2 km by 250 m) called Eyjan ("The island") in the open end of the U.

Its origin is uncertain. It might have been formed by subsidence, or it could have been formed by floods in the river Jökulsá. There are also signs of sea-water having flowed into it.

The legendary explanation is that it is a hoofprint made by Sleipnir, the eight-legged horse belonging to Óðinn, father of the gods, the crag Eyjan having been formed by the frog of his hoof.

There is a small lake at the inner end. Vegetation ranges from moorland growth near the open end to thickly growing birch and rowan trees farther in.

Ásbyrgi is a popular summer holiday destination, and is under the protection of the Forestry Service. National Forest.