Stig of the Dump by Clive King.
Stig of the Dump (1963) by Clive King is a classic of British children's literature — the story of a boy named Barney who falls into a chalk pit and discovers a cave-dwelling Stone Age boy.
Eight-year-old Barney is staying with his grandparents in the Kent countryside. He falls into a disused chalk pit and discovers a boy — whom he names Stig — living in a cave made from salvaged junk. Stig is wild, silent, wears animal skins and communicates through gestures and sounds. Despite being unable to talk to each other, they become friends.
Barney and Stig have a series of adventures: building a chimney for Stig's cave, outwitting some men who are illegally dumping rubbish in the pit, and hunting with Stig. Barney's sister Lou is sceptical of Stig's existence.
The novel ends with an extraordinary sequence: at the Midsummer solstice, Barney and Lou are transported back to Stone Age England, where they meet Stig and his tribe completing a stone monument. They help complete it. When they return to their own time, the memory fades like a dream. Barney visits the pit but Stig is gone. The adventure is over — but it was real.