More precisely, Rocky Butte is a Boring Lava Field volcano created by the stretching apart of the Willamette Valley. The Boring Lava Field became active at least 2.7 million years ago and has been extinct for about 300,000 years.
It's present shape- very steep on the east side and tapering down on the west side was caused by the impact of catastrophic Missoula Flood waters impacting the east side and stripping away massive quantites of basalt thus over-steepening the east side which caused it's near vertical east walls whice then were further steepend in the early twentieth century by quarring the exposed basalt to use for Portland's first jail which was built nearby where I-205 presently passes by Rocky Butte's eastern edge. Further quarring by the WPA in the 1930's built the wonderful retaining walls lining the roads on the butte and also the beautiful viewing platform on the summit.
The shallow valley that I-205 passes through on Rocky Butte's east side was dug out by the Missoula Flood's waters impacting the butte and subsequently passing around the butte to the north and south in a westerly direction, and those floodwaters carried away the debris which then formed a 200 foot high bar of flood debris stretching nearly 5 miles to the west directly from the west side of Rocky Butte, and that bar we now call the Alameda Ridge.
Read "Cataclysms on the Columbia" for a full desription of the Missoula Floods and their influence on Portland area topography, and read
"Ancient Oregon" for a better description of the formation of the Boring Lava Fields.
Thank's for the unrequested correction, do you have a name?
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Photo taken in Madison South, Portland, OR, USA
Misplaced? Suggest new location