Rapid development of ferricretes on a subtropical valley side slope

Ferricretes can he formed along some valley-side slopes in the southeastern United States coastal plain as a consequence of erosional exposure of zone s of iron precipitation in areas of groundwater discharge. This mode of fer ricrete formation was demonstrated due to the recession of estuarine shorel ine bluffs after hurricanes in 1996. Iron-precipitation zones exposed by bl uff retreat at Flanner Beach, North Carolina in 1996 had formed indurated f erricretes by 1998. This confirms the valley-side groundwater discharge mod el of ferricrete formation, and shows that, once the zone of iron precipita tion is exposed, ferricrete can form in less than two years. The newly form ed ferricretes also allow the identification of five distinct stages in the ir formation: ( 1) iron precipitation in the zone of water table fluctuatio n; (2) the formation of brittle iron-cemented layers; (3) exposure by erosi on or mass wasting and the first stage of hardening; (4) further hardening into indurated ferricrete: and (5) formation of limonite ferricretes, and i mpregnation with manganese oxides. The results from Flanner Beach show that the process may proceed from stages two and three to four and five in less than two years, suggesting that only short periods of stability following erosion or mass wasting episodes are necessary to allow ferricrete formatio n.