Extensive shell ridges frame the edges of marsh platforms in parts of the Biloxi Marsh of southeast Louisiana. The exact sources of the shells in these accumulations have not been clearly identified but the most likely source is a combination of shells from modern offshore and shells excavated from buried St. Bernard delta deposits. Larger or fetch-protected ridges remain stable through time, whereas ridges facing open water are more mobile, moving as much as 38 m inland from July 2017 to January 2018. Behind stable ridges, marsh platform biomass is relatively unaffected. When ridges are mobile, vegetation is smothered, leaving an exposed platform that lacks aboveground vegetation to dampen wave energy and fragments into “blocks” along its terraced edge, which in turn are deposited onshore. In the future, marshes will likely erode fastest in areas where shell ridges are mobile and remain resistant where shell ridges are stable.