b. Specific Processes: South Bird Island Quadrangle. A schematic cross section of 

 northern Padre Island (South Bird Island Quadrangle) is shown in Figure 2; Figures 3 and 4 

 are aerial photos of the northern and southern parts of the island. The geological processes 

 operating in the northern 10 miles, are discussed by Hunter and Dickinson (1970), and 

 Hunter, et al., (1972). In this area, immediately north of the north Padre Island study sites, 

 a well developed foredune ridge exists 15 to 30 feet above MSL with several maximum 

 elevations over 45 feet. The ridge is mostly continuous and is broken by only a few narrow 

 gaps. West of the foredune ridge is a plain of lower elevation, with an irregular pattern of 

 ridges and troughs. A westward-migrating, longitudinal dune field, mostly barren, lies west 

 of the plain. Bordering the Laguna Madre on the east is a relatively narrow flat strip 

 periodically inundated by wind-driven water. 



Surface configuration of the northern area was different in tiie middle to late 1800's. 

 The foredune ridge was present and similar in shape; however, the mid-island consisted of an 

 irregular pattern of vegetated dunes, while the western part of the island was a low vegetated 

 flat. There was no back-island dune field, nor a low plain in the middle part, and the island 

 was one-fourth mile narrower on the western side. During 1948-67, the migrating 

 back-island dune field advanced along the western edge of the island into the Laguna Madre, 

 an average of 700 feet; this advance is still continuing. Before 1948, the western shoreline 

 was relatively stable. 



Hunter and Dickinson (1970) attribute the present condition to the following sequence 

 of events. In the late 1800's and early 1900's, much of the vegetation was destroyed by 

 overgrazing and droughts (Price and Gunter, 1943). The previously vegetated dunes in the 

 middle of the island, and probably the foredunes, became subject to wind erosion. These 

 barren dunes then migrated westward to form the present back-island dune field. As the 

 dunes migrated past a given point, they left behind a low deflation plain, which is now well 

 vegetated. Where the migrating dunes more recently passed (since 1948), plant succession is 

 in an initial stage; this area is termed a young deflation plain. During wet cycles, the 

 westward advance of the dune field was slowed, and the trailing ridges became vegetated. 

 During dry cycles, barren sand on the trailing edge was wind-eroded to a lower elevation, 

 forming a trough which in subsequent wet years became a semipermanent pond. This 

 pattern of troughs and ridges can be traced, on aerial photos from 1937 to present, through 

 wet and dry cycles. 



Blowout dunes occur in various stages of activity along the mostly continuous foredune 

 ridge. Where the vegetative cover is locally weakened or destroyed, the wind-activated sand 

 moves northwest. Beach sand is continually added to the barren area as long as it remains 

 connected with the beach. When a new foredune is formed in its place, the sand supply is 

 choked off completely. The original active dune continues migrating westward, and leaves 

 behind a low deflation plain which eventually becomes vegetated (Hunter and Dickinson. 

 1970). Several active blowouts are evident in Figures 3 and 4. 



