The hill descends to meet the floodplain of the Columbia River, and at lower elevations, the mountain has cliffs on its northern and eastern sides. At the foot of Rocky Butte, there is a fosse (a ditch), which extends for before it opens toward the Columbia River to the north and divides to the south into two arms that run west and southwest. This depression has a width of less than and an average depth between . There is another terrace, with a height of , which sits next to Rocky Butte. It runs for to the west, with a descent of over that distance. To the south, this terrace also has a long depression near its base, which extends for from Rocky Butte. These features are likely the product of more than just erosional forces; Bretz (1925) argued that the fosse resulted from fluid eddies that pushed downward where currents interacted with the eastern side of Rocky Butte, and the terrace formed from accumulation of deposits in water under the blockage.