The Preliminary Study Of Palegeographic Changes At Baodun Site Of Xinjin

The Yangtze River basin is one of the birthplaces of Chinese civilization, thewestern Sichuan Plain, which is famous for―Land of abundance‖, even spawned ahighly developed ancient civilization. Since1995, Chengdu Institute of cultural relicsand other units has found many prehistoric settlements of4000-5000aB.P. in theChengdu Plain. Because Baodun site is the largest area, the most cultural connotationand the most representative site of those prehistoric settlements, so, the archaeologicalculture of those prehistoric settlements was named―Baodun culture‖byarchaeologists.The found of Baodun site is of great significance for the study of the origin of theancient civilization in the Yangtze River basin. At present, the study of Baodun sitehas made many achievements, but the site of the ancient environment research has juststarted.In order to further study of the origin of Baodun culture, sustenance feature,settlements structure and the change of settlements, man-land relationship, as well asthe social development and so on, at the same time,also in order to make preliminaryinvestigation for the study of the research into the origin of Chinese civilization phaseiii project, since November2009, Chengdu Institute of cultural relics have organizedarcheologists to do archaeological excavation and field investigation in Baodun site.In this paper, the stratum and remains of Baodun site were the main researchobjects. On the basis of many drilling dates, field investigations, excavationinformation and previous research results, the paper mainly study ancient rivers, depositional environment analysis and Stratigraphic classifications and so on from theangle of sedimentology. The purpose of the study is to learn the palaeogeographicfeatures of the Baodun site. Furthermore, to analysis the relationship between thepaleogeographic changes, the layout of the settlements, prosperity and abandonee ofthe site, and so on. Through above researches, we can draw the major conclusions asfollowings:(1) Combining the situation of ancient rivers survey with the drilling dates ofBaodun site, we know there are many ancient rivers in the site. We analyzed theexploratory trench of an ancient river, from the profile we can see that the ancientriver has obviously experienced three development process. We inferred the times ofthree development process of the river are late period of Baodun culture, the westernHan dynasty and the eastern Han dynasty by the method of archaeological chronology.Three development process of the river made the ancient river from big to small,finally evolved into the swamp.(2) Based on the analyses of particle size and Magnetic susceptibility on theprofile of the exploratory trench, we can see that the dual structure is obvious in thefirst process of the river and the gravel layer is at the bottom of the profile, fine sandsand silt sediments are at the middle and upper of the profile. These phenomenaconform to flood sediment characteristics. In addition, on the profile of theexploratory trench, we also found the typical characteristics of the flood sediment.There are tree trunks and leaves, which washed by the water, are found at the bottomof the river. These are direct evidences of the floods in the Baodun site.(3) We analyzed the sedimentary environments of the Baodun site by the drillingdates, the results of ancient rivers survey and the knowledge of the sedimentcharacteristics of the profile. Combined with the above information, we drew the mapof sedimentary facies of the Baodun site, and provided an important basis for furtherarchaeological excavation.(4) From the perspective of ancient climate, two global cold events, whichhappened in5200aB.P. and4000aB.P, has important significance in the beginning ofBaodun culture and the prosperity to decline of the Neolithic culture in the Minjiangriver basin. From the perspective of topography, the ancient residents made full use ofthe local topography and followed the principle close to water, but not easily affectedby the floods when they constructed the Baodun site.(5) Sites in the region are prone to sudden flooding because of catastrophic climate. Additionally, the slope is low in Xinjin area, and Xinjin is in a few big riverscrossed, so, compared with other prehistoric settlements, Baodun site is moresusceptible to be flood. The sedimentology analysis and the results of ancient riverssurvey of the Baodun site provided the direct evidence that there was a flood in thesite during Baodun culture period, and further evidenced that the ancient floods haveimportant influence on the Baodun site(6) The stratum which is no culture and no remains in the Baodun site have acertain relationship with the topography and the rush of floods of the Baodun cultureperiod. The areas that the swamp deposition concentrated are consistent with theremains focused areas, and it is explained that swamp areas are close to the ancientlife. We concluded that it have a close relation to the water-using behavior of theancient residents.