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Ⅰ.引言 南口在北京西北约50公里,正位于燕山山脉与华北平原的接触线上,控守着横切这一山脉、发源于八达岭附近的南口河谷的出口。自古以来,它就是从蒙古高原下降到华北平原以直趋北京的重要孔道之一。燕山山脉在南口附近这一段亦可称为南口山脉 南口山脉成东北——西南走向,横互在怀来盆地平原与华北平原之间,宽度约15公里。它的最高脊顶拔海高度超过1,000公尺,向两方平原的边坡都很陡峭,所以地形异常险阻。
I. INTRODUCTION The southern mouth is located about 50 kilometers northwest of Beijing and is located on the line of contact between the Yanshan and North China Plain. It controls the exit across the range and originates from the Nankou Valley near Badaling. Since ancient times, it has been one of the important tunnels that plunged from the Mongolian Plateau to the North China Plain to reach Beijing directly. Yanshan Mountains in the vicinity of the south of the mouth of this section can also be called the south mouth of the mouth of the mouth of the mountain range into the northeast - southwest, cross each other in Huailai Basin plain and the North China Plain, about 15 km in width. Its top ridge is more than 1,000 meters above the sea level, and steep slopes to both sides of the plains make the terrain unusually risky.