Micro-rods have been observed in indurated carbonate horizons (30 cm thick) that overlie a periglacial chalk formation, in contact with the present-day soil (Champagne, France). They are numerous in the upper part of the hardened layers. Variations in micro-rod morphologies are related to progressive biomineralization of organic matter, transforming purely organic rods into calcite. Mineralized rods undergo diagenesis and their arrangement evolves from a random mesh fabric to recrystallized micritic platelets to microsparite. Two types of organic micro-rods have been observed: bacilliform and thread-like bacteria. Mineralogically, micro-rods are low-magnesian calcite. Crystallographically, their length is parallel to the (104) axis and they grow along the {010} plane. Micro-rods can be associated with needle-fiber calcite. They increase the CaCO3 content of the primary matrix and infill its microporosity. Their diagenetic evolution into microsparite contributes to soil layer induration.