ZHANG Yuxu;LIU Lian
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.
