Matagorda Island is a wide, sand-rich, barrier-island complex on the central Texas coast. This barrier initially formed as an intermittently emergent sand shoal which migrated landward during the late Holocene transgression and then became stabilized as the Gulf of Mexico reached stillstand. The subaerial portion of the island complex rests on a blanket of middle Holocene bay-estuarine mud which was deposited behind the landward-migrating sand body and then was overridden by it.
Following stillstand, Matagorda Island prograded Gulfward approximately 1.6 km. During this progradation, two large tidal passes, which connected the Gulf of Mexico with San Antonio and Mesquite Bays, were closed. The island was further modified by migration of Cedar Bayou several miles to the west across the island's southern end.
Two sources of sand contributed to this barrier complex. Prior to stillstand, erosion of Pleistocene strandplain sand and middle Holocene fluvial-deltaic sand which was exposed on the shelf supplied most of the sediment to the early barrier. Following stillstand, with progradation, shelf sands were too deeply submerged to be eroded by Gulf waves. Sand, discharged into the Gulf by the Colorado and Brazos Rivers and transported southwestward by longshore currents, was deposited on the beach and shoreface of Gulfward-building Matagorda Island.