Maine’s geology is a wild story of plate tectonics, island collisions, volcanos, and great changes in sea level, and is extensively documented. Since our last Ice Age wrapped up roughly 12,000 years ago, our Gulf of Maine effective local sea level has changed dramatically in part due to ocean level rise and fall, and in part due to the glacial ice weight depressing the Earth’s crust, and the crust's subsequent rebound as the ice melted. So over the last 14,000 years, geologists report an effective present local sea level of 230 feet above present local sea level to a minus 360 feet below present local sea level; sea level has been roughly stable for the last 5K years. This is to say that Portland’s coastal location varied from 50 miles out in the current Gulf of Maine to 180 miles inland from the coast.
Publications
Maine's History of Sea Level Changes
Joseph T. Kelley and Stephen M. Dickson, Maine Geological Survey Department of Conservation, and Daniel Belknap, Department of Geological Sciences University of Maine, 1996.
A paper by the Maine Geological Survey gives an overview of Maine's complicated sea-level history, which involves world-wide changes in the level of the ocean generated by melting ice sheets and more local effects.
Virtual Tour of Maine's Coastal Marine Geology
Maine Geological Survey.
A collection of images, charts and graphs with explanatory text of numerous geologic features along Maine's 3,478 miles of tidally-influenced shoreline.
Virtual Tour of Maine's Bedrock Geology
Maine Geological Survey.
A series of photographs with descriptive captions which show: Maine examples of the three basic rock types; aspects of bedrock structure; and some places where bedrock is exposed at the surface.
Videos
Watch a Billion Years of Shifting Tectonic Plates in 40 Mesmerising Seconds
David Nield, Science Alert, 2021.
A billion years of tectonic plate movement movement in a 40-second video clip. This animated graphic shows just how fluid our planet's outer skin is, as it floats around on its magnetic and liquid core.
"'Is the sea level changing, or is it the land that is moving?' That is a difficult question to answer completely, and is the subject of much ongoing research." Maine's History of Sea Level Changes