Ásbyrgi was carved by the Jökulsá river some 11-12,000 years ago. The cliff walls are formed by a compound lava thought to originate from Stóravíti, one of the largest lava shields in Iceland. Glacial whalebacks and glacial striae are found on top of the Eyjan (“Island”) in Ásbyrgi. At several locations around Ásbyrgi, glacial striae on the eastern part of the lava flow are evidence of a glacier which scoured the area to the north and probably extended into the Öxarfjörður bay. Boulders and thin glacial moraines have been found on many parts of the lava to the west beyond Undirveggur. Ásbyrgi may have formed subglacially or on the periphery of the retreating glacier. Repeated catastrophic floods of the Jökulsá river have subsequently scooped out Ásbyrgi into the form we see today. Tephrochronology indicates that the last flood roaring through Ásbyrgi occurred about 2500 years ago, shortly after the deposition of the Hekla H3 tephra.