The presence of buried soil was discovered in the Oka River floodplain in the territory adjacent to the Prioksko-Terrasny Nature Biosphere Reserve in the area of the modern location of the northernmost steppe phytocenoses. The buried soil was classified as alluvial dark humus typical medium loamy—Fluvic Stagnic Phaeozem (Loamic). According to radiocarbon analysis, the formation of the buried soil took place in the middle and late Holocene, almost completely covering the Subboreal. The distribution of iron along the profile of the buried soil, as well as the presence of sponge spicules, indicates a seasonal or periodically stagnant moisture regime. According to microbiomorphic and phytolith analyses, the vegetation cover of the high floodplain at the initial stage of formation of the buried soil was dominated by graminoid-grass associations with a small participation of forest flora. The later stages of formation characterize the meadow-forest stage with the predominance of forest grasses over meadow grasses, the presence of conifers. Pine and oak coals are present in the buried soil. An analysis of the distribution of phytoliths of the feather grass Stipa pennata L. (the most characteristic edificator of local modern steppe communities) showed that during the formation of the buried soil feather grass phytocenoses were not widespread on the floodplain of the Oka River. The signs of the presence of feather grass in individual locations (refugia) identified in the samples require further research to determine the role of steppe phytocenoses on the floodplain of the Oka River in the Subboreal.