Abstract
The Red Light Bolson lies in Trans-Pecos Texas. Two sets of earth fissures of differing age are present in the fine-grained alluvium which fills the bolson. These fissures are characterized by a continuous to discontinuous series of elongate, shallow surface depressions and a vegetation lineament. Collapse of a shallow subsurface cavity, which formed through erosional degradation of surficial soils overlying a fracture in a relatively impermeable calcrete, most likely created the observed surface characteristics. To test this hypothesis, surface and shallow subsurface investigations were conducted. The collapse depressions of the older fissures are mainly continuous and rounded in profile. They average 4.5 and 2.5 ft in width and depth, respectively, and exhibit an average width to depth ratio (W/D) of 2.18. The vegetation lineaments of the older fissures are continuous. The collapse depressions of the younger fissures are discontinuous and rectangular in profile. They average 2.7 and 3.6 ft in width and depth, respectively, and exhibit an average W/D of 0.62. The vegetation lineaments of the younger fissures are predominantly discontinuous. The surficial soils exposed in a trench excavated across a young fissure included an A horizon of clayey to silty, fine-grained sand, and a stage 111, fine-grained K horizon. An older, stage IV, gravelly K horizon was exposed below the upper calcrete. A cavity was present below an uncollapsed portion of the fissure at the excavation site. This cavity was located in the upper portion of the upper calcrete. The upper calcrete was highly degraded below the cavity. A fracture was present in the lower calcrete below the degradation zone. The bedding and grain-size characteristics of the degradation zone and fracture fill indicate that the fracture in the lower calcrete leads to the observed surface characteristics by permitting downward migration of water and soil particles. The flux of water and sediment through the fracture degrades the upper calcrete and forms the cavity, which collapses to form a discontinuous series of depressions. These depressions capture runoff which migrates downward through the fracture. The high moisture infiltration leads to prolific vegetation growth along the fissure trace.
Sherrier, Michael Perry (1994). Earth fissures of the Red Light Bolson, Hudspeth County, Texas. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1994 -THESIS -S5536.