Origin and development of the Texas shoreline. (R. J. LeBlanc and W. D. Hodgson, 1959)
<Abstract>
The shoreline of Texas forms the boundary between the northwestern Gulf of Mexico and the western Gulf coastal plain for a distance of about 375 mi. between Mexico and Louisiana. The shoreline trends in a north-northeasterly direction and generally parallels the regional trend of the coastal plain and the outer edge of the continental shelf. The seaward slope of that portion of the coastal plain which lies below 300 ft. elevation is about the same as the slope of the inner continental shelf which is less than 300 ft. deep. This slope varies from about 6 ft. per mi. S of Corpus Christi to about 2 1/2 ft. per mi. E of Galveston, Texas. The coastal features of Texas can be divided into 4 main groups on the basis of their general characteristics and origin. These are 1) alluvial valleys and deltaic plains, 2) estuaries, 3) barrier islands, and 4) lagoons. The Gulf coastal plain in Texas is drained mainly by 9 rivers and their tributaries, Each stream of coastal Texas flows within an alluvial valley of Recent age which merges with a deltaic plain near the coast. The size of the alluvial valley and deltaic plain for any one stream is proportional to the size and silt load of that stream. Large streams, such as the Rio Grande and the Brazos River, have wide alluvial valleys and broad deltaic plains and empty directly into the Gulf of Mexico. Smaller streams, such as the Nueces and San Jacinto rivers, flow in comparatively narrow valleys and empty into shallow bays or estuaries which lie behind barrier islands. These estuaries, such as Baffin, Corpus Christi, San Antonio, and Galveston bays, are elongated perpendicular to the coast. The lagoonal type of bay lies directly behind the barrier island and is elongated parallel to the coast, or normal to the estuary type of bay. Examples of this type are Laguna Madre, Aransas, Matagorda, West, and East bays. The barrier islands of Texas range in length from about 15 mi. to over 110 mi. and rise as much as 50 ft. above sea level. One of the characteristic features of these barrier islands is a series of abandoned beach ridges which are aligned parallel to the present seaward edge of the islands. Thus, the shoreline of Texas is actually a compound feature. It consists of a Gulf shoreline and a bay shoreline. The Gulf shoreline is the seaward edge of the barrier islands and deltaic plains and the bay shoreline lies at the edge of the mainland and behind the barrier islands. The Gulf shoreline is generally very regular, arcuate, and characterized by well-developed sand beaches. In contrast, the bay shoreline is quite irregular and generally devoid of sand beaches. In many places the bay shoreline is associated with low-lying Recent marshes; low bluffs characterize the bay shoreline wherever wave action is eroding Pleistocene terrace deposits. Geological events associated with the late Quaternary have controlled the origin and development of the Texas shoreline. Some features of the shoreline are of late Pleistocene age while others are definitely related to the Recent epoch. During the last Pleistocene glacial stage when sea level was lowered approximately 450 ft., the coastal Texas streams deeply entrenched their valleys and the Gulf shoreline was probably 50 to 140 mi. seaward of the present shoreline. With melting of the late Pleistocene glaciers and the accompanying rise in sea level, Texas streams alluviated their entrenched valleys. Sedimentation in these valleys did not keep pace with rising sea level and consequently the lower portions of the entrenched valleys were drowned to form a series of estuaries. The bay shoreline of Texas originated during this stage. During the standing sea level stage, which began about 5,000 years ago, the large Texas rivers, such as the Rio Grande, Brazos, and Colorado filled their former estuaries and constructed broad deltaic plains which protrude into the Gulf. The other smaller Texas rivers which carry lesser quantities of sediments are still in the process of filling their drowned valleys or estuaries. A series of barrier islands were formed along the coast between the Rio Grande and Colorado-Brazos deltaic plain and E of the Colorado-Brazos deltaic plain, giving rise to the Gulf shoreline along these segments of the coast. The abandoned beach ridges and intervening low swales and mud flats, which are well preserved on these barrier islands, clearly demonstrate the seaward growth of the islands during the standing sea level stage. Although most of the Gulf shoreline of Texas has regressed seaward during the past few thousand years, there are a few notable examples of local marine transgressions. Three of the examples are discussed. In summary, it appears that a combination of factors, including fluctuations in sea level associated with the last Pleistocene glacial stage, the seaward slope of the late Pleistocene coastal plain, the amount and location of sediments contributed to the coastal region by Texas streams, longshore currents and waves in the Gulf of Mexico, and variations in climatic conditions from E Texas southward to the Rio Grande have led to the origin and development of the modern shoreline of Texas.