and pines, but the transition area has not been mapped as a separate unit. On map sheet 7 two separate sand and mud units, those with dominantly pine trees as natural vegetation, are recognized. The two sandy mud— pine forests units have overall slightly sandier soils, which, along with the difference in vegetation, distinguish them from the sandy mud— oak forest units. Otherwise, substrate composition is similar. The transition in soil and vegetation probably results from the progressive increase in rainfall in East Texas.
The sandy mud— prairie units have fine sandy loam and clay loam A horizons and clay B horizons. These units are common in areas of Wilcox outcrop only on map sheets 1 and 2, are rare in the Wilcox outcrop on sheets 3 and 4, and occur only in the Midway Group on sheets 5 through 7. The fine-grained soils probably reflect both finer grained substrates and lesser rainfall in the southwestern parts of the environmental geologic maps. Natural vegetation consists of prairie grass and mesquite throughout map sheets 1 through 6 but changes to oaks on sheet 7. Today, areas of sandy mud— prairie units are commonly cleared for cropland or pastureland. When settlers first came to East Texas, and to some extent even now, the sandy mud— prairies were areas of prairie among thick oak forests.
The substrate of the sandy mud units was deposited in interchannel areas by Tertiary river systems. Peats were also deposited in the interchannel areas and were eventually transformed to lignite; all present lignite mining is in the sandy mud units, and undoubtedly almost all future mining will also be there. Lignite mining may, however, intersect other units where the sandy mud substrate dips underneath them.
The sand and mud units are the most variable of any of the environmental geologic units in substrate composition and in proportion of sand and mud. The variation is on a local scale, however, and is difficult to recognize, especially because of the homogenizing influence of soil development. Nevertheless, each of the sand and mud units contains thin, but permeable, interbedded sand layers. The sands give the sand and mud units important potential as aquifers, and the proportion of sand to mud in the different units determines their hydrologic characteristics.
Runoff and erosion are major processes acting on the sand and mud units. In many places erosion has cut through the protective cover of vegetation and soil to produce deep, steep-walled gullies. Clearing of the vegetative cover for agriculture has aggravated erosion and produced such extensive gully systems that large tracts of land were made unusable. Because much mining and reclamation will occur in this substrate, its erodability is an important characteristic.
Clay Mud Units— The clay mud unit has a clay substrate and develops clay and clay loam soils of high
shrink-swell capacity. The clay mud unit is rare in the Wilcox Group except south of the Colorado River. In the mapped region its major occurrence is in the adjacent, underlying Midway Group, but even in the Midway strata it does not occur north of the area depicted on sheet 4. Clay soils developed on the clay substrate are highly fertile and extensively cultivated.
Miscellaneous Substrate Units.— lron-cemented sands and muds compose the substrate of two units. In areas of the most extensive iron-cementing the substrate material is highly indurated and resistant to erosion. The resistant material forms high-relief iron cemented uplands with slopes commonly greater than 1 0 percent. Most of the few rock outcrops of East Texas consist of ironstone. The steep slopes and rocky soil limit use to rangeland. The high-relief iron-cemented upland unit grades into rolling ironstone and sand with a decrease in amount of iron-cementing and an increase in the proportion of uncemented sands. Slopes are less than 8 percent, and there are no indurated outcrops; soils are commonly red and have abundant ironstone concretions. Vegetation on both units grades from oaks and rarely mesquite (sheet 1 ) to oaks and pines (sheet 6) and oaks, pines, and hickory (sheet 7).
Surface-mined and surface-mined/reclaimed land are two related man-made units. The units are subdivided as to whether mining was for lignite (M 1) or for clay, sand, gravel, or ironstone (M 2). Some areas mined for lignite have been or are being reclaimed as shown by an overlay pattern on the accompanying maps. No areas mined for other materials have been reclaimed; most such unreclaimed areas are barren ground with little or no productive use or borrow pits or quarries partly filled with water.
Two additional overlay patterns were used to modify mapped units. One is for flood-prone land, land upstream from reservoirs and below spillway elevation. These lands could be flooded during periods of high water in the reservoir. The flood-prone land description can modify all other units, and all other characteristics are simply those of the modified unit.
The other overlay pattern indicates low terraces. Historical evidence indicates that certain low terraces along major rivers have been inundated during low frequency, high-magnitude floods. These terraces do not show geomorphic evidence of flooding, however, and cannot be mapped as floodplains. The low-terrace designation is restricted to terraces no more than 3 m (10 ft) above the identified floodplain.