That looks to me more as if the water had worn an opening in the rocks.

The water must have been very high to make such a large opening.Yes.

See! The water swirls in at one side of the opening and comes out on the other side, making a sort of horseshoe shape of the cut-out place.

Isn't this a place in which to hide, Jane McCarthy ?" cried Harriet triumphantly.
"Hurrah! The greatest hiding place in the world." "And won't the Tramp Club be amazed when they find we are missing?
They'll think their chance of winning the camera is doubtful." "Perhaps they'll think we're drowned," answered Jane, her eyes sparkling mischievously.
"A little scare will do them good," returned Harriet, the mischievous sparkle appearing in the depths of her brown eyes.

"What do you think of it, dear ?" "Fine! It's glorious.