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The Fort Rock-Christmas Lake Valley Basin is the basin of a former inland sea that existed in that region from Pliocene through late Pleistocene time.〔 〕 The Fort Rock basin maar field includes over 30 hydrovolcanic landforms spread over an area of .〔〔 〕 == Moffitt Butte == Moffitt Butte is a dissected basaltic tuff ring, in diameter and . Moffitt Butte is not associated with a lake basin, as is the case for Fort Rock and Hole-in-the-Ground, but rising magma probably encountered permeable aquifers beneath the cone. A line of tuff rings between Moffitt Butte and the Fort Rock-Christmas Lake Valley Basin are located roughly along what would have been an early drainage route between the Fort Rock Basin and the La Pine Basin. The crater floor of Moffitt Butte is about above the surrounding plain. A smaller vent and small tuff ring, in diameter, are located on Moffitt Butte's southwestern flank. The crater of the smaller vent is filled with lava that issued from a dike on its northwest edge.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fort Rock Basin」の詳細全文を読む スポンサード リンク
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