Portland Head Light, Casco Bay, Cape Elizabeth, Maine. Portland Head Light, commissioned by George Washington and in operation since January, 1791, marks the southern side of the entrance to Portland Harbor. It is the quintessential Maine lighthouse, combining in a single view Maine's ocean, islands, rocky coast, colonial history, and maritime heritage. Several days ago I posted a photo of Schootic Point (Rocky Coast of Maine) featuring "intrusive" igneous rock that welled up as molten magma from the depths of the earth and cooled into granite before it ever reached the earth's surface. The rock under Portland Head is igneous rock too. But it is "extrusive"...it was molten magma that erupted from an arc of volcanos millions of years ago. It is very fine grained gneiss made up mostly of quartz and feldspar. The US Geologic Survey has determined the age of this rock to be 471 million years old (+/- 3 million years). Put another way, this is before the Himalayas or the Alps were formed, before the Atlantic Ocean existed, before the dinosaurs lived and before the first plants had evolved. The volcanos that must have existed near here at that ancient time have long since been eroded; all that is left is a layer of volcanic debris preserved in the rocks.