Ancient Rocks Boundary

The high ground rising east of this junction belongs to the Del Norte Mountains, which run twenty-six miles from a point eight miles east of Alpine to Del Norte Gap, nineteen miles southwest of Marathon. They form the northern part of a chain of mountains crossing central Brewster County from northwest to southeast; south of the gap the chain continues in the Santiago Mountains. Mount Ord, at 6,803 feet, is the range's highest peak, and the average summit stands near 5,000 feet. Shallow, stony soils carry oak, juniper and some mesquite.

What the plaque says

Ancient Rocks Boundary The mountains toward the east are limestone reef beds deposited in the Cretaceous and Permian seas about 135 million years ago and 250 million years ago; respectively. The limestone reefs overlie deformed rocks in the Ouachita Fold Belt, which is visible along the highway to the east, near Marathon. To the south are younger limestone beds deposited about 125 to 135 million year ago. They also rest above the Ouachita Fold Belt (so named for the Ouachita Mountains of Arkansas-Oklahoma, where formation was first studied by geologists). Toward the west, northwest, and southwest are layers of lava and associated volcanic rocks 25 to 35 million years old. These were thrust upward, into and over most of the older sedimentary rock layers. Many of the molten rock masses cooled and hardened underneath, while vast quantities poured over the surface in the form of lava flows and volcanic debris up to a mile in thickness. Subsequent erosion has sculptured these highlands by removing much of the younger volcanic rock, and cutting deeply into the older sea deposits. Out of the lower, ingenuus hills grow yucca and other cacti, contrasting with juniper and pinon on the higher elevations. (1970)

Where it stands

Worth the stop nearby

• Museum of the Big Bend — 7.1 mi
  The museum of the Big Bend region and northern Mexico on the Sul Ross State University campus in Alpine, back since 2007 in its native-stone building of 1937.
• Chihuahuan Desert Nature Center and Botanical Gardens — 22 mi
  The Chihuahuan Desert Research Institute's grounds south of Fort Davis, with an 18-acre botanical garden, a cactus museum and trails into the desert foothills.
• Marfa Lights Viewing Area — 22 mi
  The roadside viewing area on US 90 nine miles east of Marfa, where people gather after dark to watch the horizon toward the Chinati Mountains for the Marfa lights.
• Fort Davis National Historic Site — 26 mi
  A frontier post of 1854 to 1891 on the San Antonio-El Paso Road, garrisoned after the Civil War by Black regiments, with its buildings restored to their 1880 look.

More markers nearby

• Brewster County — steps away
• Texas Confederate General Lawrence "Sul" Ross — 7.1 mi
• Burgess' Water Hole — 7.5 mi
• Texas Confederate Colonel Henry P. Brewster (1816-1884) — 7.8 mi