关键词: 伊犁地区, 自然环境, 黄土形成条件

Abstract: Loess is widespread at the ranges of 900~1900m where steppe and desertificaton steppe stretches in Yili area,Xinjiang.A serial of investigations about the loess distribution have been carried out.The results show that loess deposits are closely related to the conditions of physical geography.First,the Yili area belongs to a large intermontane basin that is surrounded by Tian Shan Mountains on the north,south and east.The dusts transported by westerly from the source will deposit as they enter the basin because of the speed decrease.Second,Yili area is adjacent to the desert of Central Asia.As well known,the desert of Central Asia is rich in fine particles,especially at the downstream of Yili River where the lacustrine deposit is widespread, which derives from Tian Shan of China and ultimate flows into Balkash Lake.Without question,the desert of Central Asia supplies plentiful fine fragments for loess formation in Yili area when the wind is strong enough to transport them.Third,the local climate is benefit to the dust falling.According to the data of meteorological stations in Yili area,it is similar to that of Loess Plateau that annual precipitation ranges from 200 to 500 mm at pediment and hill, but the season distribution of the precipitation is different.Spring is a rainy period and summer is relative dry in Yili area,contrasted sharply with the monsoon region where summer is rainy and winter is dry.That is to say that the more precipitation does not agree with the high temperature period.On the other hand,snowfall is account to about 33% of annul precipitation.It is obvious that the erosion caused by snow melting is weaker than runoff formed by rainfall.In addition,the humidity of soil keeps high from spring to the beginning of summer.This causes more dust to deposit and the dusts will be captured permanently. Exception for the precipitation,the direction and strength of wind is suitable for the dust falling.Controlled by the westerlies and effected by the topography,the wind speed of Yili area is slow and calm often occurs.Fourth,the well vegetation cover can trap more dusts and make them become loess quickly.Finally,under the conditions of hemi-arid climate and better vegetation covers,the＂loessification＂is prevailing in pedogenic process.All of these are the reasons why loess is widespread in Yili area.

Key words: Yili area, physical environment, loess formation