关键词: 黄土高原, 气候变化, 古冲沟, 气候阶地

Abstract: Owing to the special structure and geomorphic conditions of Luochuan Yuan, loess with athickness of 135m has been accumulated by continuous deposition over the past 2.4 million years. This period of loess accumulation spans at least 24 climatic cycles. The climatic changes no: only formed loess-paleosol sequences but also had a profound influence on other aspects of geomorphic development.In the cold-dry stages when loess deposition was fast, soil genesis was comparably weak. In this way the fine loess layers were formed, precipitation and surface runoff remained low in these cold-dry stages. The loess layer and its geomorphic expression was stable only under such climatic condition.When humid temperate climates developed, loess deposition rates were much reduced, per-miting strong pedogenesis. In this way paleosols were formed. In addition, precipitation and surface runoff were much higher in the humid temperate stages, affecting the stability of the underlying loess. This resulted in the onset of erosion and the development of gullying. Since the fifth paleosol layer was formed five erosion stages are recognized in the Luochuan Yuan region.The process of deposition and erosion in the valley of Luohe river at Luochuan Yuan region differs significantly from that mentioned above. During the humid-temperate climatic stage, loess erosion remained strong in the inner part of loess yuan, and gullies were formed. A lot of loess material was carried into the Luohe river. Because the valley of Luohe river was narrow and the runoff volume and flood levels were higher than those in the cold-dry stage, a thick deposit of alluvium was formed in the valley. When the climate entered the next cold-dry stage, erosion was weaker in the inner part of loess yuan and surface runoff decreased. The quantity of loess material carried into the river decreased accordingly. Erosion was then occurred in the alluvium. As the flood level in the cold-dry stage was lower than that in the previous humid climate, the flood level could not reach its previous heights. The alluviuin deposited in the last climatic stage then was preserved as a terrace. Locess then began to deposit on the terrace surface. It is reasonable to consider that the terraces of Luohe river in Luochuan Yuan region was formed by such climatic changes.Although the history of Luohe river differs from that of gullying in the inner part of Luochuan loess yuan, both were formed under the influence of the climatic changes. Correlation between the terraces of Luohe river and the gullies of various erosion stages is therefore possible. Such correlation is shown in the following table;

Key words: Loess plateau, Climatic change, Ancient gully, Climatic terrace