山脊壶穴不能作为中国东部第四纪冰川的证据

Potholes on Hilltops is Not the Evidence for Quaternary Glaciaer in Eastern China

中国东部有无第四纪冰川是中国地质、地理学界争论了几十年的具有重大影响的问题。山脊壶穴被一些文献作为中国东部存在第四纪冰川的重要证据。笔者认为,山脊是一直处于剥蚀中的,所以山脊壶穴不可能是古代的遗物,应当是近、现代的产物。而山脊壶穴的冰成说不能很好地解释其地质特征,冰成机制也难以令人信服。山脊壶穴是差异风化形成的,即原始的小坑或浅坑因为积存雨水,花岗岩中的长石分解加速,相对于无水的裸脊,风化速度显著加快,从而加深、扩大。在包头以北发现很浅的壶穴(4—5cm深)即已具有口小肚大底平的形态;可以看到有许多壶穴由于位于陡崖边部,因壶壁被剥蚀穿通,不能再存水,从而不再加深;在山东新泰更有一壶穴具有两阶段演化的特点;这些现象表明壶穴是逐渐扩大加深的渐变产物。观察最近揭示的北京延庆白龙潭巨型壶穴,恰位于河流拐弯且变狭窄处,为洪水时旋涡形成。所以,中国东部的这些壶穴,无论是山脊壶穴还是河谷壶穴,都不能作为古冰川的充分证据或必要证据。

Whether or not there was Quaternary glacier in eastern China is a question with a significant impact, which has been debated for decades by the geological and geographical scholars of China. Potholes were considered as the important evidence of Quaternary glacier in eastern China in some papers. However, it is considered that the hilltop is always in the environment of denudation, therefore the potholes on hilltops could not be the ancient stuff, while they should be the result of modern and contemporary processes. And the ice-induced theory could not explain the geological characteristics of the potholes on hilltops, and the formation mechanism is also hard to believe. It is considered that the potholes on hilltops are formed by differential weathering. The original small and/or sallow pit accumulated rainwater, then the feldspars in the granite began to accelerate decomposing, comparing with the anhydrous naked ridge, the weathering rate was significantly quick, leading the pit deepened and enlarged. It is found that a shallow pit （4 to 5 cm deep） in western Inner Mongolia has had typical characteristics of potholes, i.e. small mouth, large belly, and flat bottom; many of the potholes located on the margin of the cliffs were no longer deepened and enlarged, because the pot walls were eroded through and could no longer contain water; and in Xintai, Shandong Province, the characteristics of a pothole on top of the Qingyun Mount show that it has experienced two stages evolution. These phenomena show that the potholes on hilltops are the products of gradually enlarged and deepened. The huge pothole in the Bailongtan （White Dragon pool） in Yanqing, Beijing was observed, the pothole is just located in the turning and changing narrow zone of the river, and it is considered that was formed by the flood swirl. Therefore, the potholes, either on hilltops or in valleys,could not be used as the evidence of paleo-glacier.