Abstract: On the basis of analysis and correlation of strata in Lanzhou area, it is regarded in thisarticle that the youngest stratum under the Arming System naffed by C. C. Young andM.N.Bian in 1936 is a layer of gray-white sandy conglomerate formed in the early MioceneEpoch, basis on the evidences of vertebrate paleontology found at some places such as thenorth slope of Gaolan Mountain, Hanjinzi, Xianshuihe etc.，while the oldest stratumabove the Arming System is the sixth terrace gravel layer of Huanghe River (Yellow River)under the Wucheng loess layer at Jiuzhoutai, formed 1.488±0.11 Ma (8.P.) dated by themethod of gypsum fission trace. Arming System, therefore, was formed in the period ofMiddle Miocene-Pliocene. Arming System is fossil eolian sand face of desert in light of many features such as largelaminae, sandstone with calcareous cement, sand grainsize compared with desert sand,micro-texture of quartz sand granule including dish-shaped concavity, curved grouve,curved edge and silica deposit, etc. Th,e type of physial environment for the formation. of Anning System may be fromsemiarid steppe to arid desert steppe, and it mainly formed by the arid climate of LateTertiary according to the parameters of environment e.g. redish sand colour, calcareouscemented sandstone, calcareous nodule, and relating to other conclusions ofpaleogeography and paleoboany. There must be a paleo- anticyclone centre in the west of Lanzhou controlling wholenorth China inLate Tertiary according to:(1) the occurence of foreset laminae in ArmingSxstem are mainly from west to east (2) the wide spreading of eolian red earth of LateTertiary in the east of Lanzhou such as Baode and Jinle formations.