Abstract: The lacustrine deposits of the ancient Sanmen Lake have been preserved in good condition in the Huangdigou profile which is located at the north of Sanmenxia Reservoir in Pinglu County of Shaanxi province.From a study of the magnetostratigraphy, the deposition of the ancient Sanmen Lake is known to have started in the middle of Gilbert reversal polarity chron, about 5MaBP, and ended in the late of Brunhes normal polarity chron, about 150kaBP. Since the sedimentary sequence has persisted from the early Pliocene of Neogene to the end of middle Pleistocene of Quaternary, it is referred to as the Sanmen group.Based on the lithological characters, the sequence can be subdivided into three members, the upper member consisting dominantly of sand layers, the middle member of clay layers and the lower member of basal gravel.The series is now known to have the longest period of deposition that commenced earlier and terminated later than all the others.