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In total, 42 Third System forts were built along the east coast of the USA between 1816 and 1867. Because of their purpose, to defend key locations against potential British amphibious assault, their sites were limited to islands, shoals, shorelines, riverbanks or hilltops. Sited along the Atlantic and Gulf of Mexico coastlines, many are constructed upon distinctive geomorphological landforms that are related more to features influenced by Quaternary glaciation or sea-level changes than by pre-Quaternary bedrock. A selection of forts is described to illustrate the principal geological and geomorphological features that influenced their construction: at Boston Harbor in Massachusetts on drumlins, near New York City on moraine and at Fort Pulaski protecting Savannah Harbor in Georgia on saltmarsh. Further south, Fort Clinch in northern Florida, forts guarding Pensacola Bay in western Florida and the entrance to Mobile Bay in the Gulf region, and Fort Massachusetts on Ship Island, Mississippi, were all primarily situated on Holocene barrier islands. Construction, use and 1861–65 Civil War threats to these forts can be directly related to their geomorphological context. Although all are now redundant, most remain as a remarkable and highly visible aspect of the military history of the USA.

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