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Coastal Zone and Marine Ecoregion Terrestrial and Aquatic Habitats

Description and Location

The coastal zone is that portion of the lower coastal plain that lies seaward of US Highway 17. This region includes a small portion of the mainland but is primarily comprised of tidal marshlands and associated uplands that include large sea islands that are greater in size than 1,000 acres (404.69 hectares) and extends eastward to include barrier islands, Atlantic Ocean beaches and the Atlantic Ocean shallow continental shelf offshore to South Carolina’s 4.8-kilometer (3-mile) jurisdictional boundary. The lower approximately 32 to 48 kilometers (20 to 30 miles) of all of the state’s coastal rivers is included in the coastal zone.

The inland boundary of the coastal zone is somewhat arbitrary relative to mainland habitats, but is particularly relevant to riverine and alluvial habitats since Section 50-5-80 of the Code of Laws of South Carolina establishes boundaries for fresh and ‘marine’ waters that generally are associated with US Highway 17. These boundaries were established primarily for wildlife law enforcement concerns related to the enforcement of freshwater and marine fishery laws and regulations. The actual point at which riverine waters change from fresh (less than 0.5 parts per thousand salt) to brackish or ‘marine’ (greater than 0.5 parts per thousand salt) is highly variable, even daily, depending on the combined impacts of tides and river discharge as determined by rainfall or water releases from dams. During each approximately six-hour tide cycle from maximum ebb or low tide to maximum flood or high tide, the point of change from fresh to slightly brackish water may move several miles upriver, only to return downriver during the next ebb tide period.

The soils or surface sediments of the coastal zone (sands, silts and clays) are derived from the Appalachian Mountains and are organized into coastal, fluvial (riverine) and aeolian (dune) deposits. Most of these deposits were transported seaward during the Quaternary period, which began about 1.8 million years ago. Underlying the surface sediments is a bedrock stratum of eroded sedimentary rocks dating to the Tertiary period and the Mesozoic era, between 130 and 1.8 million years ago. With the exception of manmade quarries, the bedrock stratum is only exposed within the coastal zone in banks and bottoms of rivers, in deep scoured tidal channels and on nearshore Atlantic Ocean continental shelf bottoms as “hard bottom.” The oldest sedimentary rocks are deeply buried sandstones, shales and siltstones from the Cretaceous epoch and up to 130 million years old. Limestone ranging in age from 100 to 30 to million years overlies the sedimentary rock (Matthews et al. 1980).

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