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. Eruptive history Evarts et al. (2009) assign Rocky Butte an age younger than 500,000 years. One Argon–argon dating experiment determined an age for Rocky Butte lava of 285,000 years ± 16,000 years ago, which matches the normal magnetic polarity for the lava deposits at the Butte. There is a magnetic anomaly about northwest from Rocky Butte in Portland, but it is unlikely to be associated with the volcano or the Boring Lava Field in general. Local newspaper sources report that Rocky Butte is extinct. Trimble (1963) noted that an eruption at Rocky Butte covered a hill composed of Troutdale Formation rocks, which are exposed as conglomerate rock at a height of on Rocky Butte's western slope. The exposure of rock from the Troutdale Formation suggests that Rocky Butte erupted onto a topographically irregular surface and that this surface later underwent erosion. Ecology The moderate climate and ample precipitation of the region lead to rapid regrowth of vegetation on untended sites, which can hamper fieldwork in the area. Many forests that covered the area were partly cleared for agriculture, timber, or burials in the early 20th century. These cleared and burned land plots sustain rich stands of secondary forest, featuring gorse, huckleberry, nettles, poison oak, salal, and blackberry. Myriad species of fern, as well as rapid-growth deciduous trees like alder and vine maple are also frequent. Forests support stands of Douglas fir, western hemlock, western redcedar, Pacific dogwood, bigleaf maple, Oregon ash, red alder, cascara buckthorn, Pacific madrone, and Oregon white oak; within swamps and moist areas in creeks, the shrub Devil's club can be observed. Other trees that sometimes dominate forest areas include black cottonwood. Forest communities have many additional shrubs including Indian plum, western hazel, and snowberry. Ground layer plants include the