Several faults forming a north-south striking fissure swarm are found in the Þeistareykir volcanic system. The western part of the fissure swarm terminates at transtensional faults in the southeastern part of the Húsavík fault zone. This limits the Tjörnes peninsula to the south. Many of the faults veer to the northwest and merge with the Húsavík faults. The easternmost fault, which is called Skildingahólsveggur, splits into two branches southeast of Sæluhúsmúli. Guðfinnugjá, one of these branches, continues to the north, while the Sæluhúsveggur fault swerves northwest towards Sæluhúsmúli. The triple point, where the faults separate, is found in the Skildingahraun lava which erupted just over 12,000 years ago. The Þeistareykir lava flowed towards and rested by Sæluhúsveggur and Skildingahólsveggur 2400 years ago.