As a result of the «Quaternary Silts» project, two different mudstones were found included in the Libertad Formation: one, derived from Finipliocene loess, without gravel, with less than 20 % of sand, and illite as the dominant clay mineral, which is proposed to maintain as Libertad Formation. The other mudstone, derived from basic rocks with high sand percentage, ferromagnesian minerals in the sand fraction, abundant gravel (30-50 %), and montmorillonite as dominant clay mineral, is proposed to be defined as La Carolina Formation. The two formations are actually soil stratigraphic units. The mineralogical differences suggest a different concentration of trace elements, which might result in dissimilar natural fertility behavior. The current distribution can be defined from soil associations surveyed in the soil chart of Uruguay at 1/1.000.000 scale.