How old is the bedrock of Maine?
The oldest reliable age determined for a Maine rock is 647 million years old (with analytical uncertainty of 3.7 million years), for a pegmatite near Islesboro in Penobscot Bay. Interestingly, this pegmatite cuts through rocks of the Seven Hundred Acre Island Formation, so we know that that formation is older than the pegmatite, but we don’t know by how much. Individual mineral grains from the Chain Lakes massif, north of Eustis, have yielded a wide spectrum of ages, the oldest dated at about 2800 million years old (that’s 2.8 billion). These old grains are interpreted as a variety of sedimentary grains shed from an older continent and incorporated into a younger sedimentary rock. The rock itself is about 485 million years old. So while this rock is not as old as the pegmatite in Penobscot Bay, it contains within it a few tiny mineral grains that are very old indeed. The vast majority of Maine rocks are of Early Paleozoic age, from about 360 to about 510 million years old, representing
