Formed by the ancestral Columbia and Willamette Rivers, the terraces north of Johnson Creek are generally underlain by permeable sand and gravel. Three isolated hills—Rocky Butte, Mount Tabor, and Kelly Butte—rise about above the surrounding terraces. Except for Powell Butte, the terrain on the creek's north side is less steep than on the south side, which includes Mount Scott and the Boring Lava Domes.