Fossils are preserved remains of animals and plants from the past. Usually only part of the organism remains, such as bones and teeth. Some fossils preserve the original material; others form a crystal or mineral copy of the remains. In the case of fossils like Stanley’s fish, the fossil is formed when the animal’s body is covered quickly and hidden from scavengers. As times goes by, the skeleton becomes buried deeper and deeper under sediment, putting pressure on the lower layers and turning it into rock. Minerals contained in the sediment begin to replace the minerals in the skeleton, filling in any holes. This process is called permineralisation. Eventually this results in a mineralised copy of the original skeleton. This will look like a rock in the shape of an animal, or a rock with the shape of an animal caught in it.
This is just one of the many incidents in the story where the past is uncovered, symbolising the importance of history, and how the effects of the past can be carried forward into the future. As the book progresses, the history of both Camp Green Lake and Stanley’s family will prove to be very important to the plot. Actions and events from the past have repercussions that are felt for decades afterwards, and that strongly affect the characters at Camp Green Lake today. There is a feeling of fate or karma running through the book, that actions have consequences that may not be fully realised in one lifetime, but that will send positive or negative ripples into the lives of future generations.
This page about fossils explains how a fossil is formed, using a fish as an example.