North Sister and Mount Washington mark isolated volcanic centers among the highly mafic (rich in magnesium and iron) platform of the central High Cascades. About 4.5 million years ago, eruption of mafic lava filled a subsiding Pliocene depression, creating the modern mafic edifice of the High Cascades. Compared to the eruptive products at and near North Sister, lava deposits at Mount Washington have a greater abundance of incompatible elements (elements unsuitable in size and/or charge to the cation sites of the minerals of which it is included.) Hogg Rock shows more similarity to the basaltic andesite deposits at North Sister, which are poorly enriched in incompatible elements. The part of the High Cascades that extends south from Mount Jefferson to Santiam Pass includes shield volcanoes, lava domes, and cinder cones. The high elevation of Matuyama-aged rocks east of Santiam Pass, coupled with exposure of Brunhes-aged rocks to the west, imply the presence of a northward-trending normal fault.