"Shadows" suggest something ominous and frightening. Since darkness conveys the unexpected and a lurking danger, Golding is conveying the uncertainty of the boys in their hunt for the beast -- a creature that epitomizes their greatest fear. The creature lurks in their imaginations, and what makes it frightening is that they cannot actualize it -- it is a dark, pervading presence with no real identity. All that it is is a feeling of something supernatural and malevolent. The thought of actually confronting it is a terrifying one.
"Tall trees" signifies the enormity of the task the boys face. The idea of finding the beast is an overwhelming thought, and the boys are petrified. The undertaking is for only the most courageous amongst them and, in the end, it is only Ralph, Jack, and Roger who continue the search up the mountain to find the sum of all their fears. The idea that they might confront it, directly, is appalling, and this realization tests the boys' resolve to the limit, so much so, that the two leaders, Ralph and Jack, challenge each other's courage through mockery.
The title also indicates the boys' movement from the known to the unknown. In their quest for the beast, they have to move away from their familiar surroundings at their camp and venture where no one has gone before. As they proceed, they encounter many obstacles and hurdles which hinder their progress. It is an arduous task, and every obstacle serves to accentuate their fear and creates uncertainty.
The challenges the boys face bring Jack and Ralph in direct confrontation with each other and also tests their nerve. Their continued battle for leadership is emphasized by their constant bickering and their challenges to each's courage and resolve. Golding cleverly uses the obstacles and the constant bickering between them to build up to a climax.
The chapter culminates in the boys finally seeing, face-to-face, what they believed was the real-life representation of their greatest fear. They are so overcome by what they see that they intuitively flee without a thought, even abandoning their weapons. As the narrator describes,
Behind them the silver of moon had drawn clear of the horizon. Before them, something like a great ape was sitting asleep with its head between its knees. Then the wind roared in the forest, there was confusion in the darkness and the creature lifted its head, holding toward them the ruin of a face.
Ralph found himself taking giant strides among the ashes, heard other creatures crying out and leaping and dared the impossible on the dark slope; presently the mountain was deserted, save for the three abandoned sticks and the thing that bowed.
In the end, though, it is tragically ironic that Simon, the only one who knew the truth about this beast, that it was a dead parachutist, is thought to be the beast himself and is killed by the frenzied boys.