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Figure 3. Photograph 34-74-095. Transfer zone between Presidio (upper left) and Big Bend National Park, Texas (lower right). The Rio Grande takes advantage of extension along the several faults in Colorado Canyon (center of frame) that drop that region down allowing the river to pass from the Presidio graben (upper left) to the Castolon graben  (lower right) that occupies the western half of Big Bend National Park. Santa Elena Canyon is visible about one-third the distance along the tilted mesa that extends along the lower right edge of the photo. The large circular feature in the lower left is the Santana-San Carlos caldera. The circular feature along the upper right edge is the 5-mile diameter Solitario, a caldera on a laccolith.