The ”kovárvány” called reddish, hardened embeddings of the eolian sand were examined. A few 10 cm thick, lens like or irregular bodies appear in the top of dunar morphology. They consist finer quartz sand with increased amount of clayey-silty grain size fraction and 0.5–1.2% iron content. In spite of the similar mineralogical composition of the sand fraction, differences appear in the shape and the size of the grains. The upper faces of these embeddings are gently waved, the grains are well packed, with nests of limonite concretions. The lower separation surface is irregular, often with a gradual passage to the host sand. In the SEM photos of the sand grains, biogenetical corrosion and crushing were evidenced. The bonding material with touching and void filling character consists of a colloidal mixture of iron hydroxides and clay minerals, while in the host sand, „free mason“ (cementless, uncemented?) and meniscus type bonding appear. The goethite and illite neoformation in the bonding material indicate the beginning of the diagenetic processes in this sediment. Neither ascending iron ions nor iron oxi-hidroxide precipitation was evidenced in the moving zone of the groundwater table. These embeddings represent the rest of ancient forest soil, with biogenetic iron accumulation and grain crushing. Thus, during the wet periods of the Holocene, the dunes were capped with small tree groups and bushes - and later, when the vegetation was dried out, the soil cover was partly eroded and it was buried under the moving sand nappes.