Ferricretes are soil or sediments cemented together by iron oxides. In eastern North Carolina, reducing conditions often prevail on the broad, flat interfluves. Under these conditions Fe is reduced, and soluble. Groundwater flow from these areas toward the major river valleys transports this dissolved ferric iron. When the groundwater discharges along the valley side slope it comes in contact with oxygen, and the iron oxidizes to its ferrous form. These iron oxides coat whatever material exists at that location--sand, clay, etc.
Limonite ferricrete at Fishers Landing, NC. The piece at the top is about 40 cm long.
When these iron-coated materials are exposed, typically by bank or shoreline erosion, they harden into rock-like ironstones--ferricretes. Once the Fe-coated material is exposed, it can harden very rapidly; within a year or two. They are hard enough so that geologists erroneously interpreted them as sandstone layers into the 1990s. When I read about a similar ferricrete formation process in Australia, I realized that the ironstone I'd been seeing might be formed the same way; some of them were very clearly NOT sandstones.
Recently exposed (as of January 2018) unhardened Fe-coated material.
I studied these ferricretes about 20 years ago (Phillips et al., 1997; Phillips, 2000), and had an opportunity to observe the process in action recently. Shoreline erosion along the Neuse River estuary at Fishers Landing (near New Bern, NC) has recently exposed some iron-coated material that is still unhardened. However, the entire shoreline is littered with ferricrete fragments ranging from pea gravel to small boulder size.
Ferricretes scattered along eroding shoreline, Fishers Landing.
"Ferricrete hash" derived from abrasion and wave erosion of ferricretes.
Posted 19 January 2018