A geologic map of the summit depression of Haleakala, Maui Island, Hawaii, was finished by the writer in 1935 as part of the systematic geological investigation of the Hawaiian Islands by the United States Geological Survey, in co-operation with the Territory of Hawaii. The depression is 7 miles long and 2 miles wide. The highest point on the rim is 10,025 feet above sea level. Evidence is presented to support Cross' theory that this “crater” is due chiefly, and perhaps entirely, to stream erosion by the recession of two great amphitheater-headed valleys, Keanae and Kaupo, from opposite sides of the mountain, instead of being due to either collapse, sliding away of a side of the cone, or explosion, as set forth by others. Kipahulu, Waihoi, and Manawainui valleys were likewise formed during this long erosion period. The ancient main rift zone crossed the summit and the heads of Keanae and Kaupo valleys. Renewed volcanic eruptions along this rift built large cinder cones and poured out voluminous flows that nearly masked the ancient divide between the two valleys and partly filled both to the sea. The depth of the fill probably exceeds 2,000 feet at the valley heads. The lava flows displaced the drainage channels and caused the streams to undercut their valley walls, thereby widening their heads. Many of the late eruptions took place along fissures crossing the valley walls, thereby giving vent to flows that cascaded down the steep walls. A broad thick mud flow underlies the late lavas at the mouth of Kaupo Valley.