关键词: 地貌, 黄土丘陵, 形成模式, 演变

Abstract: The formation and evolution of loess hilly experiences a complicated process W ith a survey on the landforms of the typical loess hilly region in the northern Shaanxi Province the paper proposes the formation model of loess hilly and discusses its geographic evolution. W idely occurring on the Loess Plateau most primordial loess hillies take the shape on the basis of the underlying old hillies with the aggradations of the loess; while the secondary loess hillies are formed in the erosion and alteration of Loess Yuan and Loess Tableland by ravines The river terrace and layered landform of ravines in loess hilly regions reflect the formation and evolution of the rivers and ravines The primary tributaries of the Yellow River formed at the end of the early Pleistocene Epoch; at the tum of the early and late M id-Pleistocene Epoch, the main water system formed the primary and secondary tributaries of the Yellow River; at the end of the M id-Pleistocene Epoch such major ravines as Jiuyuan Valley took their shape; and by the end of late Pleistocene Epoch the structure of waterways and ravines was quite similar to that of present days Most of those gulches that did not cut into the bedrock yet then formed in the Holocene Epoch and most gullies with a length ranging from several to hundred meters took their shape in these historical epochs or the latest decades

Key words: landform, loess hilly, formation model, evolution