Not Scarlet But Gold by Malcolm Saville Lone Pine 1978 Paperback GD

The clue to the treasure - "Not scarlet but gold"

"I can't forget those words," said Jenny. "I know they sound silly, but I think I ought to know what they mean. The trouble is that I can't quite -"

"Look! Look, Jenny!" Harriet screamed. "Something is happening up there!"

Tom was the first to realise their danger. He grabbed the twins and pushed them back down the track.

"Run!" he shouted. "All run back. The dingle is moving. It's a landslide!"