How are tidal holes formed?

Yin Chao Cave is located in front of Zizhulin Temple in the southeast of Putuo Island, at the foot of Longwan, and refuses to go to Guanyin Temple. The cave is semi-submerged in the sea, with a depth of about 30 meters, and the depth from the cliff to the bottom of the cave is about 10 meter. The grotesque rocks in the cave are scattered and the dog's teeth are overwhelming. The coast here is winding, with rugged rocks and rugged rocks. The bottom of the cave is open to the sea, and there are two gaps at the top, which are called skylights. In the thirty-eighth year of Kangxi in Qing Dynasty (1699), the cave wall was engraved with the imperial book "Yin Chao Cave".