“Mount Agamenticus is located within the exposed upper part of the “plumbing system” that supplied lava to a very large volcano that was active about 220 million years ago”…”the summit is not the volcano; it is, along with Second and Third hills, an erosion-resistant part of one of the circular dikes that once supplied lava to the eruptions.”
“When this volcano was active, North America was still attached to Africa, and according to current interpretations a strip of land along the Maine coast and extending out to the edge of the continental shelf is a piece of Africa that remained “stuck” to North America when the continents began to move apart, forming the Atlantic Ocean. Mount Agamenticus is located within a long line of volcanic structures which extends more than 1000 miles seaward as a line of conical submarine volcanoes; landward, the line continues northwesterly through southern Maine and New Hampshire into Quebec.”