In the innermost part of Hvalfjörður (Whale Fjord), a buried hyaloclastite ridge can be seen. The ridge occurs in the middle of Múlafjall mountain, which separates the Botnsdalur and Brynjudalur valleys.
The hyaloclastite ridge in Hvalfjörður formed in a volcanic eruption under an ice sheet during an early Pleistocene glacial period. The division between Pliocene and Pleistocene records the shift from Tertiary (warm conditions) to Quaternary (glacial periods) 2.5 million years ago. This boundary can be found on the coast in front of Múlafjall. The hyalocastite ridge, which is oriented in a northeast direction like many normal faults in the area, is best exposed in Botnsdalur, where it can be seen rise up in the middle. Subsequent to its formation, lava from shield volcanos has flowed up against the ridge and finally covered it.
The best locality to observe the ridge is near the abandoned restaurant in Botnsdalur valley, where there is also large parking area. From there is a pleasant walk up to the ridge exposure and to the top of Múlafjall mountain.
Sigurður G. Kristinsson, 2010