Belknap's volcanic deposits have well-preserved pressure ridges (tumuli) and levees on their surfaces, and they were emplaced between 7,000 and 1,300 years ago. Tephra erupted from Sand Mountain Volcanic Field and Belknap forms a thick blanket across Santiam Pass and near McKEnzie Pass, with radiocarbon dating ages of 3,440 ± 250 to 1,600 years B.P. for deposits from source vents, making them younger than Mazama Ash deposits. Cinder cones near Inaccessible Cone have been nearly buried to completion by basalt and basaltic andesite erupted from Belknap. Belknap's ash deposits are recognizable by their dark color among road cuts and at near the base of lava flow margins.