The Boquillas Formation is one of many amazing geological features of Big Bend National Park. Many layers of sedimentary rock have been pressed together, then folded by the action of subterranean earthquakes and faults, then exposed by erosion.
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" . . . Along the west side of the Sierra del Carmen (fig. 4) is a series of "white cliffs" that extend from near Dog Canyon southward to the Rio Grande. The "white cliffs" are formed mostly of Boquillas Formation (table 1, p. 27). The black rock layer seen in some of the "white cliffs" is a sill (fig. 24C, D) that intruded the Boquillas Formation prior to faulting. . . .
FIG. 37. The Boquillas Formation. This flagstone unit consists of thin-bedded limestone and shale. The interbedded shale layers are soft and easily eroded and leave the harder layers in prominent relief much like the weathered appearance of an old brick wall. The formation has many fossils (figs. 7-9).
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Many incidents of human interest have been recorded in or near Boquillas. The village gave its name to the Boquillas (flagstone) Formation (Udden, 1907, pp. 29-33) that crops out along the west side of the Santiago-Sierra del Carmen ranges from near Persimmon Gap southward to the Rio Grande and westward up the river to beyond Mariscal Mountain (fig. 4). Boquillas was the headquarters camp of Arthur A. Stiles, topographic engineer for the United States Geological Survey, who made the topographic map of the Terlingua and Chisos Mountains quadrangles, part of which are in the Park. It was here in December 1903 that Bill Ernst, storekeeper and Justice of the Peace, called a meeting of local settlers to recommend names to be used on the Stiles map (Madison and Stillwell, 1958, pp. 5-14). Ernst's name was used for a tinaja (rock water hole), a valley, and a tank that are in the area where he ranched. The term Ernst is used by Maxwell et al. (1967) to designate the lowest member of the Boquillas Formation. Bill Ernst was fatally shot only a few miles from Boquillas, and the switch-back curve in the old road south of the road tunnel is known to many as Ernst curve or "dead man's" curve.
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Presidio de San Vicente.—Presidio de San Vicente, about 10 miles up the Rio Grande from Boquillas, was built by the Spaniards during their campaigns to conquer and Christianize the Indians. Madison and Stillwell (1958, p. 50) reported that the Presidio was built in 1774. It was in ruins in 1857 (Emory, 1857, pp. 74-92); all that is now visible are small mounds of rock and adobe. Presumably, the chapel was a prominent structure and there was a stockade for the prisoners, but the individual buildings cannot be identified. The site is on a gravel terrace about 50 feet above the Rio Grande, some 2 miles southwest of the old abandoned San Vicente village in the Park. This Presidio gave its name to the mountain at the west, to the village (fig. 46), and also to the upper member of the Boquillas Formation, as the flagstone of that unit crops out near by. These Upper Cretaceous rocks crop out in the structurally low area that lies between the Sierra del Carmen on the east and Sierra San Vicente (mountain) on the west.
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FIG. 50. Ruins of the old Mariscal (Lindsey) mine and part of Mariscal village as they appeared in 1963. During the World War I years, this was a flourishing community with several hundred inhabitants. Folds of the Boquillas Formation are shown and some of the ledges are sills in the Boquillas."