Abstract: The 1st stiff clay layer in the Yangtze River delta is buried paleosol in the flood land of the Yangtze River and its formation experienced many sedimentary and pedogenic processes. The main iron-manganese nodules in the stiff clay layer are rhodochrosite, siderite and limonite. Based on the graph of Eh-pH relationship of iron-manganese, the best advantage for the formation of nodules of rhodochrosite and limonite is Eh<0 and pH>8. The time of the pedogenic process was in the last ice age, therefore this area was colder and drier in climate and lower in sea level than today.The main chemical process in the soil was oxidation,a period of the formation of oxide minerals such as limonite.With the deposition and covering of the marine facies layer in the postglacial period, the soil turned into diagenetic process.Consequently the chemical process in the soil was dominated by reduction and this process was the main development period for nodules of iron-manganese carbonate to form.