Hydric soils derived from some red parent materials are “problematic” to identify during wetland delineations because they resist redox-induced color changes. These (PRPM) soils can be identified using the F21 – Red Parent Material field indicator, but the distribution and cause of the phenomenon, remains uncertain. The objectives of this study were to identify locations where PRPM occurs for appropriate use of the F21 field indicator throughout the country, and to better understand why PRPM soils resist redox-induced color changes. We found that PRPM is associated with sedimentary, hematite-rich, “red bed” formations and the deposits derived from them. Guidance maps have been developed showing where use of F21 is appropriate to support hydric soil (and therefore wetland) delineations impacted by PRPM. We also demonstrated that the cause of PRPM appears to be related to larger crystallite sizes of hematite in PRPM soils.