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Mount Bachelor is a basalt to basaltic andesite summit cone that sits atop a shield volcano 15 km southeast of South Sister. It is the largest feature of the many eruptive vents that make up the Mount Bachelor volcanic chain, a 25-km-long, north to south aligned chain of about 50 vents. The majority of the eruptive activity along the volcanic chain occurred in 4 discrete episodes, which are defined on the basis of paleomagnetic data, between 18 and 8 ka. The total volume of the chain is estimated at about 40 km3, with the volume of Mount Bachelor estimated at about 25km3 or more than 50% the volume of the entire chain. Glacial striations about 1 km (0.6 mi) north of the Mount Bachelor suggests that the flow of ice was not obstructed by the mountain during the latest glacial maximum (22-18 ka), the Suttle Lake stade; however, the cone was almost completely built by the time a subsequent glacial advance occurred, between 12.5 and 11 ka as evidenced by glacial deposits on the volcano edifice.