A volcanic mountain with older Tertiary rhyolite breccia and metabasalts in lower part, capped by a basalt lava succession erupted mainly during subglacial activity about 500 thoushand years ago.
Park the vehicle on side road Eyrarsveitarvegur, on the north side off Route 54, shortly after crossing the bridge over Kolgrafafjörður. Hike up gully named Markgil. Lower part is scree composed of rock fragments of hydrothermally altered basalt (greenstone), rhyolite breccia , and basalt intrusive sills and dikes. In upper part of gully there are outcrops of these rocks, part of the Setberg volcanic center of Tertiary age. Near top of gully there is a rock ledge, composed of young basaltic pillow lavas, breccia and basaltic tuff. This is the Klakkur formation, a product of a subglacial eruption about half a million years ago. This formation creates a cap over the entire mountain of Klakkur. It also occurs in Eyrarfjall to the north, where the Steinahlíðarhaus volcanic plug is the likely source. This formation is also present on top of Kirkjufell to the west.
On north part of Klakkur is a small pond, possibly in an old crater. North of pond there are outcrops of massive basalt lava flow, glaciated. Descend to northeast of the lake, down the Oddalækur gully that emerges at the site of the old Hjarðarból farm, now the site of two summer cabins. The gully contains a several outcrops of the rhyolite breccia and Tertiary basalts. Return to road to southeast end the hike.