meander scars that are larger than those of modem rivers, crop out in terraces above the Trinity and 
San Jacinto floodplains. Although the Pleistocene streams appear to have been larger, the geologic 
processes forming Beaumont and Deweyville deposits resemble those active today in the vicinity of 
the Galveston Bay System. 
The Holocene units on the geologic map (5) are like the Pleistocene deposits described above, 
except for one anthropogenic unit, fill and spoil. These man-made deposits, readily seen at a scale of 
1:250,000, occur along Buffalo Bayou and the Houston Ship Channel, formerly in the San Jacinto River 
floodplain, in the Texas City Dike, in much of Pelican Island, and behind the northeast end of Gal¬ 
veston Island. Very fine sand, silt and clay compose the Quaternary alluvium in the valleys of the 
Trinity and San Jacinto Rivers and the bayhead deltas where the rivers empty into the bays. Slightly 
finer sediments dominate minor stream valleys, because these streams derive their load from the 
surrounding Pleistocene sediments. Fine sands containing some shell are the principal sediments 
composing two modem barrier islands, Galveston Island and Bolivar Peninsula. 
Aerial photographs, cores and radiocarbon dates permit reconstruction of the geologic history of 
Galveston Island (6). Linear ridges and swales nearly parallel to the present shoreline are clear 
evidence of the seaward accretion. The barrier is composed of fine sand at the surface, which becomes 
finer both deeper and seaward, and beds dipping seaward slope increasingly less at depth (Figure 
4). Maximum thickness of the well-sorted, relatively pure barrier sand is about 30 feet. The basal 
strata are approximately 5,300 years old. Galveston Island formed a narrow sand bar and enlarged 
with the seaward accretion of offlapping fine sand (Figure 4). Because of the thickness of the deposit, 
the attitude of the bedding and bulwarking of underlying stiff Pleistocene clays, a relatively stable 
barrier island results, in contrast to the less stable barriers of the east coast of the United States. 
Bay Geology 
Researchers at the Texas Bureau of Economic Geology (7) described the geology of the bay floor 
using samples collected on 1-mile centers in the bays and about 1 mile apart in tidally affected 
streams. Sampled sediments came from a thin veneer overlying the coarser Pleistocene/Holocene 
sediments that filled entrenched valleys during the sea-level rise. Samples were classified on the basis 
of relative percentages of gravel (shell and rare rock fragments), sand and mud (silt and clay). Mud 
composes the largest expanses of the bay, especially in the deep bay centers (Figure 5). Gravel (shell) 
is more common in very shallow water and adjacent to shorelines. Gravel (shell) and sand occur only 
in the highest energy environments; both are more abundant near the shorelines and in shallow water 
affected by storm waves. Oyster reefs form the only other sediment type in the bays. Because of their 
high calcium content, they are comparable to limestones in older rocks. 
In addition to measuring the textural characteristics of the bay sediments, researchers conducted 
multi-element chemical analyses on most samples (Table 1). Total organic carbon was measured 
separately. Thirty major and trace elements were analyzed spectrographically, of which 11 elements 
were reported. These selected metals—barium, boron, calcium, chromium, copper, iron, lead, man¬ 
ganese, nickel, strontium and zinc—are useful for understanding the geology of the bay and for 
detecting anthropogenic impacts on the bay. Largest boron concentrations (+148 ppm) occur in bay 
muds having the highest total organic carbon. Manganese also associates with greater organic carbon 
concentrations in fine-grained sediments (400-1,800 ppm). Highest strontium concentrations (>1,000 
ppm) are in oyster reefs. Because metals are frequently associated with industrial pollution, those are 
reported separately in Table 1 with natural levels versus contaminated sediment values. 
Bay sediments have probably been affected by salt diapirism and/or faulting, but satisfactory 
data are unavailable; thus, the effects of these processes on bay geology cannot be assessed. 
Climate 
The Galveston Bay System lies within the warm part of the temperate zone of the Northern 
Hemisphere. Texas climate is controlled by (1) latitude, (2) proximity to the Gulf of Mexico, (3) winds 
blowing gulfward from Pacific and Arctic frontal systems, (4) decreasing elevation north and west 
to south in Texas, and (5) a position west of the Bermuda high-pressure cell (8). The Galveston area 
has a modified maritime climate controlled by the Gulf of Mexico and is classified as subtropical- 
subhumid. 
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