Encyclopedia of Texas Seashells: Identification, Ecology, Distribution, and History

Chapter 1: Shells in Texas Coastal History

5 chapter  Shells in Texas Coastal History kim withers For millennia,Texas mollusks have been important as food, and their shells have been fashioned into tools and ornaments and used as building materials.They even lit streetlamps in Houston during the 1860s, when a gas was produced by burning oyster shells together with coal. Today, some species continue to provide tasty meals, but their shells are mostly collected by amateur hobbyists (conchologists) and professional malacologists, the scientists who study shells. Only slightly above sea level, the vast Texas coastal plain gives way to offshore Gulf waters that deepen gradually over the broad, shallow continental shelf (Fig. 1.1). Along the coast, windswept beaches face the Gulf on sandy peninsulas and long barrier islands where beachcombers can search for shells thrown onshore by wind and surf (Fig. 1.2). Wetland and shallow submerged habitats such as seagrass beds and oyster reefs within the enclosed lagoons and bays support a wide variety of interesting and edible mollusks, as well as Figure 1.1. View of the Texas coastal plain in relation to the Edwards Plateau to the north and the rest of the Gulf of Mexico coast and continental shelf. CHAPTER 1 6 become the contiguous United States, glaciers covered most of the upper Midwest, New England, and parts of Montana and Washington. Sea level was 91–137 m (299–449 ft) lower than today (Curray 1960), and the Texas shoreline was about 80 km (50 mi) east of its present position (Tunnell 2002a; Fig. 1.3). At that time, there were no bays; rivers in deeply cut valleys flowed directly into the sea. Estuarine development was probably very limited because estuaries are both smaller and rarer during glacial periods than during nonglacial periods (Day et al. 1989). During the last 1 million years, sea levels similar to the relatively high levels of today and extensive estuaries existed only 10%–20% of the time. On geologic time scales, estuaries are ephemeral and expand or disappear quickly in response to changing sea levels. Between 10,000–12,000 and 18,000 years ago, the last ice age waned and sea level began to rise. Although fluctuating up and down slightly over the next several millennia, sea level continued to rise until it reached essentially modern levels between 3000 and 4500 years ago. As sea level rose, it drowned the deeply cut river valleys on the Texas coast and formed the bays and estuaries that exist today. Once sea levels stabilized, sandbars and shoals that had formed offshore began to merge (Weise and White 1980). Pleistocene river-delta and barrier-island deposits , laid down when sea levels were low, eroded; and waves, currents, and rivers carried sand to the sandbars. The sandbars emerged as a chain of short barrier islands located between the Late Pleistocene river valleys that served as tidal passes leading to the bays and lagoons behind the newly formed islands. The barrier islands continued to grow through “spit accretion” as sand carried by longshore currents was deposited on their downcurrent ends; eventually the many short islands merged into the few long islands that are present today. Although erosional /depositional processes on parts of the barrier islands of fish, crustaceans, and birds. These marine and estuarine environments have provided and continue to provide the rich, diverse habitats needed as the long history of shell exploitation continues in Texas today. Geologic History of the Texas Coast The role of shells in Texas coastal history begins with the coast and the processes by which it was formed. The coastline we see today is largely the product of geologic and geomorphologic processes begun at the height of the last Pleistocene glacial period about 18,000 years ago.Then, the majority of Earth’s water was locked up in the ice sheets that covered all of Greenland and most of Canada, Alaska, and northern Eurasia. In what would Figure 1.2. Beachcomber on Padre Island, the longest barrier island in the world. Photograph by Kim Withers. Figure 1.3. Evolution of the Texas coastline starting at the glacial maximum 18,000 years ago to about 3000 years ago when sea level was essentially the same as today. Modified and redrawn from Weise and White (1980). Shells in Texas Coastal History 7 of small, isolated outcrops that extend south from Penascal Point (Baffin Bay) for at least 10 km (6.2...