Postglacial, mafic eruptions are more common in the Sisters Reach — which includes Belknap — than anywhere in the Cascade volcanic arc. A lava flow lies next to South Cinder Peak, the Nash Crater-Lost Lake cone cluster, Sand Mountain Volcanic Field, Inaccessible Cone chain, Blue Lake Crater, and a number of monogenetic scoria cones and chains. The McKenzie and Santiam Pass area saw more than a dozen distinct mafic eruptions between 4,500 and 1,100 years ago according to radiocarbon dating, which corresponded to a pulse of mafic eruptions in the late Holocene epoch. Other nearby mafic eruptive units occur at Sims Butte, Cayuse Crater, LeConte Crater, Mount Bachelor, the Egan Cone cluster, and the Katsuk-Talapus chain, which likely were all emplaced between 18,00 and 8,000 years ago. At the south of Sisters Reach, there is a postglacial, basalt lava flow that lies on the eastern flank of Sitkum Butte (a cone that is older than this lava flow). The recent eruptive activity at Belknap Crater means that it is one of the youngest mafic volcanoes in the Oregon Cascades.