As Brian is trying to figure out his situation, he decides that he ought to check around the lake to see what he can see and on the far side he finds a spot that will be particularly useful as a shelter. He believes it is lucky that he ever decided to search in the first place and that it led him to this very useful rock formation.
He describes the place as looking like part of the rock had been "scooped by something" to make a perfect smooth overhang that is just about tall enough for him to stand in. It is lucky because he will only have to wall off a small part of it to give him protection from the worst of the elements. He felt fortunate to find someplace that would be waterproof and work better than anything he knew how to construct on his own.