北京西山灵岳寺古冰川沉积物何在?

NO EVIDENCE FOR THE ANCIENT GLACIAL DEPOSITS IN LINGYUESI AREA OF WESTERN MOUNTAIN, BEIJING

北京西山灵岳寺附近的第四纪沉积物不是真正的古冰川沉积物。该处山腰凹地中一套以粗大砾石为主的混杂堆积是较典型的复式稀性泥石流堆积体,其时代为晚更新世之前,而不是晚更新世末期产物。所谓晚更新世末期的冰川堆积体和冰蚀地貌,实际上是不存在的。晚更新世时期,北京西山地区属冰缘环境,而非冰川环境。

Lingyuesi lowland is located in the centre of the Western Mountain, Beijing, at the height of 810-950 m a-s.l. Zhao Sunling and others (1983) proposed the existance of the ancient glacial deposits, and indicated some glacial geomorphological features and glacial deposition-al types, such as paleocirque, ancient snowline and glacial trough, surface moraine, laterial moraine and terminal moraine.In 1986 and October of 1989, the authors observed and inspected the so-called 'glacial evidence' in Lingyuesi area. But the facts we found show that the Zhao's glacial hypothesis is untenable. The reasons are as follows:1. The so-called ancient cirque and horn distributed on southern slope of Huangcao range (at about 1442 m a.s.l.) in northern part of Lingyuesi lowland bears no any resemblance to glacial cirque and horn, and there no evidence for former glaciation can be found. They are the snow-eroded depression and cryogenic bulge.2. The so-called glacial red-yellowish boulder clay widely distributed in Lingyuesi lowland bears no any resemblance to glacial deposits The boulders in the deposits are of higher roundness and being will sorted, indicating that they are mainly produced by alluvial processes The sediments were carried by the water in alluvial channels and torrents rolling down into the lowland for the formation of the debris flow.3. Sporopollen composition analysis, the existance of periglacial ice wedges, and ther-moluminescene dating show that there a savanah steppe or tundra environment prevailed in the Lingyuesi area during late Pleistocene. Therefore, it is unlikely to believe that the snow-line had ever depressed so low (1100m a.s.l.) to meet the development of glaciation in Lingyuesi lowland.From the evidence on the changes of botany, snowline and periglacial climate on the basis of the permafrost it is inferred that no glaciation was ever in the Lingyuesi and other areas of Western Mountain, Beijing. Therefore, periglacial environment existed there rather than glaciers.