How does the author use foreshadowing in The Crystal Cave?

The technique of foreshadowing is used often in this novel to introduce us to a situation or character that will be of importance later in the novel. This is done in the case of the couple and the cave, which we gather we will find out more about later in the story. In Chapter Seven, for example, the mood is intense, and we can sense some future event or foreshadowing with the scene that Merlin witnesses. The setting of the cave can almost be seen as a door to another world.