Why does the writer say 'the rocky island' instead of 'a rock island'?
This is the beginning of a book:
"Look, Dad!" said Rachel Walker. She pointed across the blue-green sea at the rocky island ahead of them. The ferry was sailing towards it, dipping up and down on the rolling waves. "Is that Rainspell Island?" she asked.
Her dad nodded. "Yes, it is," he said, smiling. "Our holiday is about to begin!"
I'm wondering why the writer says 'the rocky island', not 'a rocky island', in the second sentence, even though they refer to the island for the first time. Is that because there is only a rocky island ahead of Rachel and her father?