Flipping through field notebooks…
A widespread volcanic field covered most of the Southern Rocky Mountains in middle Tertiary time, 40 to 25 m.y. ago (approximately Oligocene time). This field covered an erosion surface that beveled structures formed during the Laramide orogeny in Late Cretaceous and early Tertiary time. The source vents from which the volcanic rocks were derived were largely restricted to the deformed area. Recognized volcanic centers lie mostly within a broad triangular area bounded on the east by the Rocky Mountain front, on the northwest by the northeast-trending Colorado mineral belt lineament, and on the southwest by the southern margin of the recurrently active Uncompahgre-San Luis uplift. Local volcanic centers existed also in the Never Summer Mountains and Rabbit Ears Range north of the mineral belt lineament. The resulting volcanic field thus consisted of a major southern segment covering all of south-central Colorado and adjacent New Mexico and a northern segment extending into the mountain areas of north-central Colorado. The two segments were linked along the trend of the Colorado mineral belt.
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