Frodo and his friends are leaving the Shire secretly by going west into the Old Forest, instead of north to the road. The Forest is reached through a break in the Hedge used to keep the unfriendly trees out of the Shire …

“It had walls of brick at the sides, which rose steadily, until suddenly they arched over and formed a tunnel that dived deep under the Hedge and came out in the hollow on the other side.

“It was dark and damp. At the far end it was closed by a gate of thick-set iron bars. On the far side was a faint path leading up on to the floor of the Forest; but it vanished as soon as it brought them under the trees.”
— The Lord of the Rings, Book I, Chapter 6