The 400-mi-long Texas shoreline is characterized by barrier islands separated from the mainland by lagoons,

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bays, and estuaries up to 8 mi wide. Regional studies indicate that barrier morphology and texture and composition of beach sediment, although largely unrelated to modern rivers, are related to the distribution of sand-rich late Pleistocene facies on the inner continental shelf. For example, Matagorda Peninsula, near the Brazos River, is narrow, receding, and has a high oyster-shell content.

Narrow, regressive barriers occur where Pleistocene strand plains are absent, where Pleistocene deltas are mud-rich, and in Pleistocene interdeltaic areas. These regressive barriers have a high shell content (dominantly estuarine species), and varying amounts of caliche, siderite, beach rock, and sandstone fragment gravel. Beaches retreat 7-40 ft/yr in erosional areas. Dunes are rare on narrow barriers, and shell ramps extend several hundred feet bayward ending abruptly as steep avalanche faces.

Terrigenous sand is the dominant sediment type of wide barriers such as Matagorda Island; no modern stream contributes sand to this barrier. Broad barriers develop where sand-rich Pleistocene deltas and strand plains are present and the sand budget is large. Morphologic features of these barriers are fore-island dunes, beach ridges, and broad barrier flats. Beach ridges, indicating rapid accretion, are characteristic of the older barrier segments. Today, fore-island dunes, suggesting cessation of accretion, are relatively well developed on these barriers.

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