Thrainsskjoldur is an extensive lava mound north of Mt Fagradalsfjall.  There are no resent remains of volcanic activity on its top, but vast lava areas around it suggest great volcanic activity most of the way around it during the recent Holocene epochs.  Among its products is the lava field surrounding Mt Keilir, the hills Keilisborn, and almost overrunning the former Mt Small Keilir.  The communities of Vogar and Vatnsleysustrond are located on top of the lava field of the craters around the Thrainsskjoldur Shield Volcano, which was one of the most active and productive volcanoes of the Reykjanes Area.