The Big Bend region contains many areas that are commonly called “badlands.” Badlands are areas where soft sedimentary rock has been exposed and heavily eroded by wind and water. The Lakota called the topography “Makhóšiča,” meaning literally bad land, while French trappers called it “les mauvaises terres à traverser” – “the bad lands to cross.” The term badlands is apt: badlands contain steep slopes, loose dry soil, slick clay, and deep sand, all of which impede travel and other uses. Badlands form in semi-arid or arid regions with infrequent but intense rain-showers, sparse vegetation, and soft sediments: a recipe for massive erosion.
The Javelina Clays, a late Cretaceous deposit, comprise most of Big Bend’s badlands; one of the best areas to see them lies at the western extremes of Big Bend national park, and continues through Study Butte west to Terlingua. The clay here is mostly bentonite and is varicolored in shades of yellowish brown, pink, grey, green, and maroon.
The formation is closely allied with the Aguja Formation, which is made of yellowish grey to dark brown medium-grained sandstone interbedded with yellowish-brown and maroon clay. It is fine textured and appears to have no particular structure.
The area is interesting to geologists and paleobiologists because dinosaur bones and silicified and agatized wood fragments are widely scattered throughout the material.
Photographers like the area because of the unique way in which these clay deposits weather and because of the many colors to be found in the landscape here. The clay is impervious and weathers characteristically into rounded topographic forms.
A geologist named J. A. Udden wrote the best description of the area I have ever read:
They are so fine in texture as to be quite impervious to water. Inversely they will not yield enough moisture to enable plants to grow, except where their surface has been mixed with or covered by some land drift.
When rain falls the surface of the bare clay swells up into an exceedingly sticky mud, which renders the land practically impassable to man and beast. Pools of water will stand on the ground after heavy showers and they will evaporate away by heat and sunshine while only a small part of the moisture filters into the clay. When the ground dries, the clay shrinks and cracks extensively, but as the moisture only affects the upper one or two feet, the cracks are limited to the same depth. The clay retains the moisture with such tenacity that the outer layers of a moist lump will warp and break off while the kernel is yet somewhat plastic. As a result, the drying clay breaks up into irregular, angular, hard lumps, less than an inch in diameter. These cover the unweathered strata beneath to depths of from one to three feet on hills and slopes where the clay is bare. They are hard and tough, sometimes wholly separate from each other, and sometimes partly adhering. With every rain the process is repeated, the lumps swell up and are again dried and warped. The swelling as well as the warping produces a small creeping motion among the clay lumps, small in extent but evidently powerful. On slopes gravity aids all movements in a downward direction and counteracts all other movements. In the long run the accumulated effect of this influence results in a motion in the direction of the slope. The whole bed of clay lumps thus creeps forward like a glacier. The movement is evidently very slow, but many of the clay hills show unmistakable indication of its reality in rounded flowing contours.
Another feature which gives visual interest to the area is the presence of numerous intrusive dikes arising vertically from the surface. The soft clays are quickly washed away, leaving behind tall walls that look positively constructed but whose purpose is both unknown and unknowable.
In its forms, textures and colors, the Javelina clays present a visual feast for photographers. They are certainly worth visiting if you’re in the area.