Complex sentence: This marked the third phase of creation was the peak period of creation of the Grottoes, and it is estimated that 60% of the caves seen at Longmen came about in this period from 626 till 755.
Split into simpler sentences: This marked the third phase of creation and the peak period of the gottoes' creation. It is estimated that 60% of the caves seen at Longmen came about in this period from 626 till 755.