A special type of rock suite was discovered in the lower part of the Member 3 of Shahejie Formation of Paleogene in the Shulu Sag of Central Hebei Depression, Hebei Province, which is neither typical carbonate rocks nor classic clastic rocks. Based on detailed observations of cores and thin sections combined with comprehensive analyses of testing data, it is known as the hunji rock. Hunji rock (in broad sense) as well as the concept and system about mixing deposits were introduced into the classification of sediment type and lithofacies and thus provide proofs for scientific definition of marl. According to their origin, hunji rock can be divided into three types: textural mixing, interbedded mixing  and interlayered mixing deposits. Each type can be subdivided into several kinds: textural mixing deposits include rocks mainly composed of terrigenous components and rocks that mostly consist of intrabasinal composition. Interbedded mixing deposits is the most important types of rocks. Laminated microcrystalline limestones are mainly composed of limymudstone, marl and micrite which  make up light and dark ribbons with organicrich materials sometimes. The arranges of lamina are various and can be subdivided into 4 types in terms of the different contents of composition in situ and allogene composition. Interlayered mixing deposit consists of  thicklybedded conglomerate (carbonate breccia) intercalated with the former two types of hunji rock. Tectonics, climate and provenance are the principal factors for controlling the formation of Hunji Rock.