sediments as well as fine-grained materials in the lagoons and on parts 

 of the open shelf. Very fine and fine grained sand comprises the beaches 

 and dunes . 



In the study area, marine nearshore processes have been dominant in 

 forming the barrier islands and in modifying the physiography of the 

 region. Galveston Island is probably the best known example of a bar- 

 rier island that has prograded seaward from about 3,500 to at least 800 

 years ago, based on research by LeBlanc and Bernard (1954), Bernard, 

 LeBlanc, and Major (1962), Lankford and Rehkemper (1969), and Bernard, 

 et al . (1970). Both Galveston Island and Bolivar Peninsula are character- 

 ized by elongate, abandoned beach ridges separated by low swales which 

 parallel the present shoreline. The ridges indicate the islands have 

 accreted seaward. The source of this sand is thought to be the Sabine 

 River and other rivers to the east (Bernard, Major, and Parrott, 1959). 

 Figure 11, which shows a cross section of Galveston Island based on 

 borings, indicates the maximum thickness of Holocene sands occurs in the 

 center of the island. The Pleistocene erosion surface underlies the 

 island at about -12 meters (40 feet) and has a gentle seaward slope. 

 Figure 12 shows that the Pleistocene surface remains at about the same 

 elevation under the eastern part of Galveston Island, but at Bolivar 

 Roads there is a deeply incised river channel with a thalweg depth of 

 -31 meters (-120 feet) filled with Holocene sands and muds. Rehkemper 's 

 (1969) interpretations of a number of borings show that this channel is 

 the ancestral Trinity River which was deeply incised during Pleistocene 

 low stands of sea level. 



III. RESULTS 



1 . Continental Shelf Morphology . 



The Continental Shelf width varies from a maximum of 232 kilometers 

 (145 miles) off the Texas-Louisiana boundary to about 112 kilometers 

 (70 miles) off Matagorda Bay. As shown by the 18.3-meter contour in 

 Figure 3, considerable change in sea floor slope occurs in the study 

 area between High Island and Freeport . The inner shelf off High Island 

 is extremely flat and featureless with a slope of 0.2 meter per kilometer 

 (1.7 feet per mile); off Freeport, the 18.3-meter contour is 12.6 kilo- 

 meters (7 miles) offshore and the shelf is considerably steeper with a 

 slope of 1.5 meters per kilometer (9.2 feet per mile). 



2. Shallow Subbottom Structure and Stratigraphy . 



The morphology and geologic character of the sea floor and Coastal 

 Plain in the study area are directly related to past and present geologic 

 conditions such as erosional and depositional processes, uplift and sub- 

 sidence, sources and quantities of sediment supply, and the worldwide 

 rises and falls of sea level due to fluctuations in the climate. 



a. Planar Reflectors and Channels . The CERC seismic profiles con- 

 tain several very prominent reflectors which are traceable over much of 



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