Huerfano Park, in south-central Colorado, is bounded on the west by the Sangre de Cristo Mountains and on the east by the southern end of the Wet Mountains (Pl. 1). At its south end, the park opens onto the plains near the termination of the bordering Wet Mountains uplift, but, at the north, it merges across a low divide into the Wet Mountain Valley, with which it is, in part, structurally continuous. Huerfano Park itself is of particular geologic interest, because there are Eocene sedimentary deposits of Green River and lower Bridger age, equivalents of which are not found elsewhere along the eastern front of the southern Rocky Mountains. Thrusts of the Sangre de Cristo Mountains intersect these remnants of Eocene sedimentary rocks along the western border of the park, and thus record some later stages of mountain-building that might, otherwise, be scarcely distinguishable from much earlier stages of . . .