manganese concretions

We're doing an archeological survey that traverses the terrace of the Schuylkill River in Berks County, PA. In some excavation
units located within a portion of the terrace mapped as the Philo series (Fluvaquentic Dystrudepts), a high density of manganese
concretions/nodules occurred in the Bw2 horizon (silt loam), within an A-AC (coal overwash)-Ap1-Ap2-Bw1-Bw2-Bw3 profile; the
Mn nodules were densest between about 70-90 cm below surface. Any concensus on how much time these Mn nodules take to
form? Also, do the concretions become larger with age, and would the horizon correctly be termed a Bwc? Thanks.