Collapse is the dominant geological disaster in rocky mountain area, and usually many individual-perilous rock or bulk-perilous rocks exist in the source of collapse. The formation mechanism of perilous rock is one key link for hazard mitigation of collapse. According to the field investigation, four types of perilous rock at the source of Mt. Hongyan in the national scenic spots of Simianshan are classified as the falling perilous rock, the toppling perilous rock, the sliding perilous rock and the counterfort perilous rock, with the percentage of 70%, 15%, 8% and 7%, respectively. The general geological conditions to trigger the collapse disasters in the Mt. Hongyan are concluded aslandforms with four cliffs, rock composite with sandstone and mudstone alternatively, and three sets of rockmass structure planes with the orthography. Field observations have identified the pacemaker of rock cells in mudstone below cliff, the weathering velocity of mudstone in rockcell under pressure is about 5.2 cm/a, 2~6 times exposing in air. Further, based on the geomorphology the chained principle for the developing of collapse in Mt. Hongyan is revealed. The result shows that collapses are easy to take place when the length of rock cell is over 3 m.
