These shields are mostly circular in form, built up from pahoehoe lavas and composed of a low-sloping lava cone surrounded by an lava apron; the older shields are made from picrite, the younger, bigger ones from olivine-tholeiite. They probably were formed in long-lived eruptions (years to decennies). The best known edifices are Selvogsheiði (height 176 m, basal width 4,8 km, summit width 0,7 km, volume 0,64 km3, ; 8,200 years old acc. To A. Hjartarson) and Þráinskjöldur, 12500 years old, volume 5,2 km3, Heiðin há, 7,500 years old, vol. 6 km3, Sandfellshæð, 12,800 years old, 4,5 km3 Sandfellshæð is a very regularly constructed shield volcano and the biggest in the southern part of Reykjanes Peninsula.Another important shield volcano on Reykjanes Peninsula is the around 5.000 years old volcano Leitin.