BIG BEND The Impact of Human Use Upon the Chisos Basin and Adjacent Lands NPS Scientific Monograph No. 4

Structure and topography

The geologic structure of the Chisos Basin (Maxwell et al. 1967; Maxwell 1968) is comprised of a series of horizontal layers of Cenozoic-age volcanic ash, sandstone, conglomerate, and lava overlaying Cretaceous rocks. These two series were elevated by igneous intrusions during the Cenozoic Era. Erosion, sapping. and weathering eventually left resistant caps of lava and intrusive ridges which formed a hollow bowl. Rocks within the basin consist of Upper Cretaceous limestone (especially along the northern base of Ward Mountain), shale, and sandstone. In many areas the floor of the basin is composed of transported rock debris or gravel.

The resistant lava caps and intrusive domes create a very sheer, steep relief around the inner perimeter of the basin. These slopes are dissected and provide natural passes into the basin, such as Panther Pass and the Window. The drainage pattern is simple, with all of the debris and runoff from the basin eventually passing through Window. There are three major courses of heavy water flow, two of which meet just before entering Oak Creek to pass through the Window. One of these receives drainage from the Pulliam-Bailey and north basin region, while the other drains the Mt. Emory and east slopes of Ward Mountain. The third enters the latter and provides drainage for Roger Toll and areas south of Juniper Flat to the northern ridge extending from the base of Mt. Emory. A minor water path drains the Upper Basin complex from Casa Grande to Juniper Flat and eventually enters the Pulliam drainage near the Ward-Pulliam junction. The Pulliam-Bailey path drains the area which is most affected by man: the Upper and Lower Basin complexes.

Because of the steep terrain, large areas on the upper slopes are actively moving talus slopes. Some of these talus slopes have become stabilized and are covered with vegetation, while others are still in the process of stabilization. The many eroding ridges and the dissected floor of the basin have resulted in many different substrates, slopes, elevations, and aspects, thus providing a variety of habitats for the vegetation.

Soils

The soils of the West Texas mountains have been classified by Carter (1931) as belonging to the Brewster series. These soils, primarily derived from igneous parental material, are stony, noncalcareous, friable in texture, and range in color from red to red-brown. The soils of the Chisos Basin are complex because the parent materials are variable and were variously mixed during their erosion and transport. Because the topography is steep and complex, the distance of transport and degree of mixing resulted in localized differentiation. This differentiation has in turn been influenced by the vegetation and its role in soil-building.

For this study soils were collected at 18 sites (Table 1, Fig. 2) in conjunction with vegetation samples in order to determine soil-vegetation trends. The soil used in analyses represents a sample collected at the center of the vegetation transect from the 0 to 6-inch depth. At the time of soil collection, the percentages of rock (>22 mm), gravel (2-22 mm), and sand (<2 mm) were determined by sieving all excavated materials and weighing the particle classes to determine percentage of the total. Particle size is important in the damming effect it has upon runoff waters. The larger particles provide a greater damming effect, permitting the water to remain longer over a given area, thereby increasing infiltration and soil moisture.

Fig. 2. Map of soil collection and vegetation sampling sites: ors, old road scar; als, Aloysia lycioides site.

Table 1. Location and topographic characteristics of the sites sampled for soil and vegetation in the Chisos Basin.

The sand component of the excavated material was placed in paper bags and allowed to dry before storage. Upon return to Norman. Okla., the soil was sieved (2 mm) and transferred to an air-tight jar for storage. All analyses on the air-dried soil were based upon oven-dried equivalents and reported in terms of percent per gram.

The pH was determined by the standardized glass-electrode procedure of Piper (1942). Texture was determined by the Bouyoucos method (Bouyoucos 1936). Total organic carbon was determined by the Walkley and Black method (Piper 1942). A gasometric method by Jackson (1958) was modified for quantification of calcium carbonate. Total phosphorus was determined by the method of Shelton and Harper (1941). In all cases duplicate samples were analyzed.

The results of soil analyses are presented in Tables 2 and 3. The site analyses are divided into the three major vegetation formations for presentation. All of the vegetation was rather distinct with respect to formation with the exception of sites 5, 8, 9. and 12. Because an attempt was made to acquire data on opposing exposures on similar soil types and at equivalent attitudes, several diverse and intermediate vegetation types were obtained. The above four sites fit this category; the first two because of impact, and the second pair because of exposure. They were then categorized by potential development.

The substrate from which the soils were derived was not important in contributing to soil color (Tables I and 2). The substrates can be grouped into three major types. Sites 9 and 10 were located on the Boquillas limestone at the northern base of Ward Mountain. A second substrate is composed of large, sharp, igneous rocks still in relatively organized position with respect to each other. These, evidently not greatly weathered stones, dominate sites 1-4, 7, and 14-18. The third substrate type consists of small, smooth, igneous rocks which show signs of great mixing and weathering. Sites 5, 6, 8, and 11-13 possess these. Neither the vegetation formations nor the soil-class types are confined to a particular substrate. The soil class, based upon texture, is no different for the vegetation formations and thus forms a mosaic distribution pattern. The third substrate type is more common at lower elevations in the basin, whereas the second type is common in Upper Basin and higher elevations.

The results of the chemical characteristics of the soils (Table 3) indicate trends with vegetation types, but only one outstanding correlation with substrate. On the limestone substrate, as would be expected. calcium carbonate was above 25% per gram in both sites. Soil pH increased as vegetation cover decreased or as a more xeric type was encountered. Calcium carbonate showed a similar trend even when the radical values were removed from the formation means. The amount of total phosphorus is nearly double in the woodland compared to the desert although the values for each greatly overlap. The amount of total carbon is not significantly different between the woodland and desert formations, but greatly increases in the chaparral formation. This may be due to a more rapid and constant incorporation of plant material into the soil in the chaparral, as the leaves of the dominant chaparral plants are generally much smaller than those of the other two formations (Table 4).

More work on vegetation and soil relationships in the basin and park must be done before a pattern develops. Data to be discussed and included in this study will be an added component to this picture, but only for intermediate elevations.

Table 2. Physical characteristics of soils sampled in the Chisos Basin.

Table 3. Chemical characteristics of soils sampled in the Chisos Basin.

Table 4. Summary of the dominant plants in the three formations in the Chisos Basin with their respective Importance Value. Generic names can be found in Table 6.