Mt Hestfjall (317 m) rises gradually higher from the south to the north in the Grimsnes County and is almost totally engulfed with water.  Its highest point is called Hesteyru (The Horse’s Ears).  It is a hyaloclastite mountain with a dolerite shield created during the Ice Age.  Some scientists consider it a section of a shield volcano, which subsided mostly, but others compare its creation with the Surtsey Island (1963-67).

In the gradually sloping southern part of the mountain signs of a much higher sea level during the end of the Ice Age and the beginning of Holocene are prominent as elsewhere in the District, even up to 110 m above the present mean sea level.  The visibility from the mountain is excellent on a fine day.

According to the legend a huge monster occupies a big cave under the mountain.  It usually keeps quiet and stays in its cave, but when it leaves, the whole volume of River Hvita disappears into it and the river dries up down below.  Such incidences nowadays are explained by the ice dams, which sometimes cause floods in the flat Skeid County during winter.