flood plains and drowned channels of the San Jacinto and Trinity Rivers 

 which were inundated as sea level rose during the past 20,000 years. 

 Galveston Island extends about 48 kilometers (30 miles) from the inlet 

 at Bolivar Roads to San Luis Pass. West of San Luis Pass, Follets Island 

 continues for 20 kilometers (12 miles) to the Brazos River Inlet channel 

 and includes the Surfside Beach community near Freeport. About 48 kilo- 

 meters of coast in the Surfside-Freeport area and west to Matagorda Bay 

 is a headland region which protrudes into the gulf and represents an 

 ancestral delta complex formed by the Brazos and Colorado Rivers. This 

 is one of the few areas along the entire 592 kilometers (370 miles) of 

 Texas coast which is not fronted by barrier islands. 



3. Geologic Setting and Regional Stratigraphy . 



Galveston County lies within the Coastal Plain Province of the Gulf 

 of Mexico geosyncline. Of greatest importance to this study are the 

 Pleistocene- and Holocene-age sedimentary deposits which make up the 

 youngest of the Coastal Plain deposits (Fig. 9). These materials are 

 composed primarily of alluvial, deltaic, estuarine, and marine deposits. 

 Lankford and Rehkemper (1969) suggest that two of the primary factors 

 controlling the distribution, geometry, and composition of these deposits 

 are (a) tectonic uplift of inland areas and subsidence of offshore gulf 

 areas, separated by a hinge line; and (b) fluctuating elevations of sea 

 level during the past million years or more in response to periods of 

 worldwide glaciation and deglaciation. These sea level fluctuations had 

 a major influence on forming the geologic character and present physiog- 

 raphy of the Texas mainland coast and inner gulf shelf. Periods of 

 depressed sea level initiated episodes of mass erosion on land as rivers 

 eroded deep into the valleys and tended to meander more across broad 

 flood plains. In contrast, as sea level rose base levels of the streams 

 were also raised and deposition of sediment predominated. Evidence of 

 different episodes of geologic conditions are sometimes obvious on land 

 in the form of deeply buried ancestral stream channels, broad thick 

 fluvial flood plain deposits, extensive deltaic deposits associated with 

 many of the major streams, and relict barrier spits, islands, dunes, and 

 strand plains resulting from past sea level elevations higher than the 

 present one. 



a. Montgomery Formation . The earliest formation of direct importance 

 to this study is the Montgomery Formation (Upper Lissie) (Fig. 10) which 

 is the name for materials deposited during the Sangamon interglacial 

 period directly preceding Wisconsin Glaciation. The Montgomery Formation 

 is composed of various sedimentary facies that reflect fluvial, deltaic, 

 lagoonal and open marine environments of deposition and range in composi- 

 tion from gravels and sands to sandy clays and clays (Anderson and Clark, 

 1977) . During Wisconsin-age (the last glacial episode) the Montgomery 

 Formation was subaerially exposed and subjected to erosion and weathering 

 processes which altered some original sediments to ferruginous sands and 

 stiff oxidized clays with ferruginous concretions. 



b. Beaumont Formation . The Beaumont Formation (Fig. 10), which is 

 younger than the Montgomery and unconformably overlies it, is the most 



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