<P> The Sea Islands are a chain of tidal and barrier islands on the Atlantic Ocean coast of the Southeastern United States . Numbering over 100, they are located between the mouths of the Santee and St. Johns Rivers along the coast of the US states of South Carolina, Georgia, and Florida . </P> <P> Settled by indigenous cultures over thousands of years, the islands were selected by Spanish colonists as sites for founding of colonial missions . Historically the Spanish influenced the Guale and Mocama chiefdoms by establishing Christian missions in their major settlements, from St. Catherine's Island south to Fort George Island (at present - day Jacksonville, Florida). Both chiefdoms extended to the coastal areas on the mainland . The Mocama Province included territory to the St. Johns River in present - day Florida . The mission system ended under pressure of repeated raids by English South Carolina colonists and Indian allies . Spain ceded its territory of Florida to Great Britain in 1763 following the British victory against the French in the Seven Years' War, which resulted in Britain taking over all French territory east of the Mississippi River . </P>

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