It's pretty evident there is no opening between the great rocks or we must have found it.

We must examine the smaller bowlders.

They may have one so placed that it can be dropped down over the entrance.

That flat slab is a likely-looking place, for instance.
Three or four of you get hold of it and heave it up." The men gathered round to lift it.

Ralph stooped down and peeped under as they did so.
"Hurrah!" he shouted, "there is an opening here." Several of the others now got hold of the stone.