2022.7.26備課
why was dad neverous before they set off?
why was dad neverous before the ride homel?
what should we do for safety when we ride a bicycle?
壺穴是在河流上游經(jīng)常出現(xiàn)的一種地理特征。由于雨水令河水流量增加,帶動上游的石塊向下游流動,當(dāng)石塊遇上河床上的巖石凹處無法前進(jìn)時(shí),會被水流帶動而打轉(zhuǎn),經(jīng)歷長時(shí)間后將障礙磨穿,形成一圓形孔洞,稱為壺穴。