The rocks and sediments of Maine have preserved a dynamic history stretching from more than a billion years ago to present day. A large portion of Maine's bedrock was formed during the Paleozoic era, hundreds of millions of years before the first dinosaurs. The loose, unconsolidated materials that cover Maine today were deposited much more recently - only 12,000 to 20,000 years ago during the last Ice Age. For a good summary of the geological history of Maine, see here.