中文摘要：

通过对陕西北部典型黄土丘陵区地貌调查，提出了黄土丘陵地貌形成模式，并对地貌演变进行了探讨。黄土高原原生黄土丘陵分布广泛，主要在下伏古丘陵基础上由黄土加积而成；次生黄土丘陵是黄土塬、黄土台塬等经沟谷侵蚀、改造而成。黄土丘陵区河流阶地和沟谷层状地貌反映了河流、沟谷的形成与演变。黄河一级支流形成于早更新世末；早中更新世末、晚中更新世初黄河一级支流和较大的二级支流形成了较完整的水系；中更新世末如韭园沟等较大的沟谷形成；晚更新世末河网、沟谷格局与现在已基本一致；尚未切入基岩的冲沟多形成于全新世；长度百米至数十米的切沟多是历史时期或近几十年来形成的。

英文摘要：

The formation and evolution of loess hilly experiences a complicated process. With 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. Widely occurring on the Loess Plateau, most primordial loess hiUies take the shape on the basis of the underlying old billies with the aggradations of the loess ; while the secondary loess hiUies are formed in the erosion and alteration of Loess Yuan and Loess Tableland by ravines. The fiver 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 turn of the early and late Mid-Pleistocene Epoch, the main water system formed the primary and secondary tributaries of the Yellow River; at the end of the Mid-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.