information from the core and grab sample sites to adjacent sea floor 

 areas. The TBEG map actually contains 38 sediment descriptors; however, 

 such detail was not appropriate for this study so the sediments were 

 grouped into the four primary classes (Fig. 13). Detailed descriptions 

 of each core are contained in Appendix A, and textural analyses of the 

 sand fractions are included in Appendix B. 



The Wentworth scale for soil classification was used for describing 

 sediment textures and size classifications in this report. Table 1 shows 

 that classification and the Unified Soil Classification (USC) , as well as 

 the relationship between grain-size diameters in millimeters (mm) and phi 

 ((|)) units. 



b. Mud . The most common sediment type found in the study area is a 

 mixture of silt and clay termed mud in this report. (Mud constitutes all 

 materials with grain diameters smaller than 0.063 millimeter or > 4 phi.) 

 The mud occurs over much of the study area seaward of the shoreface (Fig. 

 13). When wet, the mud is generally greenish-gray to dark gray in color, 

 soft, and fairly cohesive; however, the mud becomes denser and harder 

 when dried. In most places, the mud occurs as a horizontally stratified 

 veneer and secondarily as fill material in several of the buried ancestral 

 stream channels which transect the shelf. Evidence of a high degree of 

 bioturbation, a low shear strength and high moisture content, and a rela- 

 tionship of the mud to deeper deposits on the seismic records and in the 

 cores suggests deposition by modem hydraulic shelf processes. 



c . Sand . Sand recovered in the cores does not exhibit much diversity 

 in compositional and textural character. It is predominantly quartz, gen- 

 erally of very fine to fine grained size (0.063 to 0.25 millimeter, 4 to 



2 phi), and is poorly to well sorted. Most clean sand (free of fines) 

 appears moderate to well sorted, and is found only in the shoal area 

 adjacent to the Galveston south jetty, the ebb tidal shoal at San Luis 

 Pass, and parts of the shoreface region of Bolivar Peninsula and of 

 Galveston Island west to within about 2.4 kilometers (1.5 miles) of 

 Surfside Beach (Fig. 13). Its presence in this area is based on infor- 

 mation in Fisher, et al . (1972, 1973). The width of the shoreface sand 

 body (distance from shore to the sand-mud boundary) varies from about 

 0.4 kilometer (0.25 mile) off central Galveston Island to about 2.5 

 kilometers (1.6 miles) off San Luis Pass and the eastern end of Galveston 

 Island. The sand width in other areas averages about 1.6 to 2.4 kilome- 

 ters (1 to 1.5 miles). East of Galveston Inlet to High Island, clean 

 sand is noticeably absent from any of the CERC cores or the TBEG grab 

 samples. 



d. Stiff Clay . Greenish-gray to reddish-brown and yellow stiff clay 

 was found in 22 of the 34 cores and in several grab samples. The clay 

 generally appears leached and oxidized and locally contains ferruginous 

 and calcareous nodules. Its physical character, relationship to overlying 

 modem sediments, and position and configuration on the seismic records 

 suggest that the top of the clay is an erosion surface that was exposed to 

 subaerial processes before sea level rose to present levels. As shown in 



32 



