再论北京西山山地壶穴的形态特征与成因机制

Revisiting the geometrical characteristics and genesis of hillside potholes in the Western Hills,Beijing

北京西山"京西古道"的"驮兽蹄窝",近些年来因为进山旅游的人越来越多而名声大噪,常见诸报刊、电视、网络等媒体。至于其真正成因,却鲜有学者深究。通过对出现"驮兽蹄窝"的北京西山牛角岭、石佛岭和峰口庵三个垭口详细的野外观察和实地测量,认为基岩路面上那些圆形、椭圆形或多边形凹坑,是历史上无数次暴雨流挟带砂砾冲刷与磨蚀基岩坡面上原先的构造和岩性薄弱部位而逐渐形成的山地壶穴(hillside potholes),属于正常的雨流侵蚀现象(rain-induced erosion)。壶穴的分布特征(在山脊垭口两侧,沿基岩路面上沟槽成串分布)、几何形态的非对称性、内壁的水平擦痕、底部的中央岛现象,以及长、短轴长度、长短轴比(a/b)、宽深比(D/h)和长轴倾向的统计结果与D-h相关性等数据,皆证明"驮兽蹄窝"的成因是不可能的。

In this paper,we revisit and add data to our earlier investigations on potholes which were drilled into the bed rock on the slopes of stony mountain where rainfall produces a brief abundance of water that transports sediments（ pebbles,granules and sands） to erode deflation hollows through abrasion,corrosion and dissolution. The hillside potholes in the region of the Western Hills（ Beijing,China） was interpreted by previous authors as hoof holes and taken for granted as evidence of an important ancient trade route during the Ming and Qing Dynasties（ 1368-1912 AD）. The interpretation was based on a baseless speculation that thousands of horses have stepped into the same but progressively deepening holes during several hundred years because by nature,horses instinctively avoid stepping in deep（ ≥5-10 cm） or muddy-water-filled potholes in order to prevent any possible injury to their ankles of legs. Based on geometrical analyses of the hillside potholes measured from three sites（ Niujiaoling,Fengkou’an and Shifoling） in the Western Hills,this study provides a linear relationship between depth（ h） and mean diameter（ D） of the potholes： D = Nh ＋ M,where Mis a critical size of the initial concavities（ seminal potholes） that subsequently underwent growth,and N is the ratio of diameter expanding（ wall erosion） speed to deepening（ floor abrasion） speed. The M-N data from the Western Hills lie consistently on a trend curve（ N = 2. 46 M-0. 47,R2= 0. 60） obtained from stream,marine and hillside potholes measured from other 34 sites in the world,suggesting that the depressions are indeed the rainwater-induced hillside potholes rather than hoof holes.