Texas Bureau of Economic Geology Padre Island National Seashore: A Guide to the Geology, Natural Environments, and History of a Texas Barrier Island

MAN-MADE AND MAN-MODIFIED UNITS

Besides the natural environments of Padre Island and Laguna Madre, there are some environments formed or modified by people. Three categories of spoil (M1, M2, and M3), which is the material dredged from channels through Laguna Madre or the barrier island, have been mapped on plate. Land modified by human activity (M4) includes the areas covered by the Malaquite Beach development and petroleum exploration and production sites.

Spoil (M1, M2, M3)

Sediment dredged to produce and maintain the Intracoastal Waterway, Mansfield Channel, Yarborough Pass, and numerous smaller channels in Laguna Madre has been piled into mounds adjacent to the channels. Most of the spoil heaped above the water surface (M1) subsequently became vegetated (fig. 107 and pl. I, M1 photograph). Like the natural Bird Islands, some of these man-made islands have become important bird nesting sites, particularly in grids T-2, U-2, and N-1 (pl. I). Many spoil islands have also become popular havens for fishermen. Cabins built on these spoil mounds by sportsmen now require a use permit from the State of Texas.

Figure 107. Dredged spoil along the Intracoastal Waterway (pl. I, grids V-2 and W-2). The central heap is surrounded by reworked subaerial spoil and subaqueous spoil. Grasses have begun to migrate onto the subaqueous spoil from the adjacent grassflat. View is to the southeast.

Spoil is generally composed of sand, mud, and shell, but it varies considerably. The original composition depends on the substrate through which the channel is dredged. Later subaerial (above water) and subaqueous (underwater) reworking may winnow out the finer sediment. Some channels were dredged completely through the thin layer of recent sediments and into the more compacted Pleistocene sediments below. Consequently, pieces of hard, cemented Pleistocene sandstone or coquina (cemented shell), the deepest materials to be dredged, occur on the tops of many spoil heaps.

The spoil mounds are reworked by wind, rain, and lagoonal waves and currents. For example, wind has reshaped some of the larger subaerial spoil mounds that have dune fields developed on them in areas where no vegetation exists. This reworking spreads out the spoil and lowers the elevations of the heaps. Reworked subaerial spoil (M2) surrounding the central spoil heap (M1) (fig. 107) is often flooded by wind tides and reworked by lagoonal currents and waves. Thus, this reworked spoil, covered in many places by algal mats, may be considered the man-made environmental equivalent of wind-tidal flats. Where dredged sediment has been piled onto wind-tidal flats, as in the Land-Cut Area, the spoil around the edges of the heaps has undergone natural processes so long that it is almost indistinguishable from the tidal flats.

Subaqueous spoil (M3) is the third spoil category mapped on plate. When spoil is dumped into lagoon waters, some of the sediment spreads out below the water surface and can immediately be classed as subaqueous spoil. With time, erosion of originally subaerial spoil contributes reworked sediment to the subaqueous environment. Most of the subaqueous spoil is devoid of marine grasses, but in some areas, species that prefer the generally shallow environment have migrated from surrounding grassflats onto the spoil (fig. 107).

The M1 unit also includes made land in several places where spoil dredged nearby or fill material brought in from elsewhere was used to build land out into the lagoonal environments to serve as subaerial earthen piers (grids E-3, F-3, G-3, and X-11). Although all subaerial spoil is a kind of man-made land, the term made land refers only to that land which is built with the primary purpose of creating new land area above sea level.

Land Modified by Human Activity (M4)

Areas mapped on plate I as land modified by human activity (M4) include a variety of features designated on plate I by either labels or symbols. Most of these features are land modifications necessary either for petroleum exploration and production or for construction of facilities to serve National Seashore visitors. The modified land areas are relatively small compared with most natural environments. Most modified lands lie in the northern part of the National Seashore where development for Seashore visitors has taken place. Modified land was mapped where the alteration of natural environments was evident on aerial photographs.

The Malaquite Beach development (grid Q-4) was carefully planned and constructed, and is maintained without harmful effects to the surrounding natural environments. Areas indicated as modified land at Malaquite include the visitor facilities, parking lot, sewage treatment ponds, and paved campgrounds.

Several petroleum production sites (fig. 108) and some abandoned exploration sites occur on Padre Island (examples in grids W-4 and K-4). Exploration and production activities have caused little disturbance of island or lagoon environments; devegetation scars left at the end of drilling projects are small and have either healed quickly or remained stable without much wind erosion and damage to adjacent environments. Land modifications on Padre Island were made for specific purposes and were controlled so that the natural environments would not be endangered. However, carelessness and abuse of the land and vegetation can cause considerable damage by altering the balance of natural processes in the environments.

Figure 108. Petroleum company facilities on the northern part of the Seashore (pl. I, grid T-4). View is to the northwest. Corrals and sheds of the Novillo cattle line camp are visible just beyond the facilities.