By about 12,000 years ago, most of coastal Maine had emerged from the sea. Today, the blue-gray clays of the Presumpscot Formation are found at elevations as high as 300 feet above sea level. Even so, the land is still depressed about a hundred feet below its pre-glacial elevation. The indented character of the Maine coast reflects a shoreline of recent submergence. If you look at a chart of Muscongus Bay, you can see how the parallel ridges and valleys have been inundated, leaving chains of islands that trail in a southwesterly direction away from the mainland.
Modern vegetation is generally a reflection of climate, soils, and past land use.
The vegetation in the watershed is transitional between the coastal spruce-fir ecosystems to the east and a forest dominated by white pine and red oak to the southwest. The vegetation of the coastal zone, extending about five miles inland from Pemaquid Point, differs markedly from the forests north of The Narrows. The dominance of boreal trees like balsam fir and white spruce and birches on the islands and exposed peninsulas south of The Narrows can be attributed to summers with fog and onshore breezes blowing over the cool waters of the Gulf of Maine. Inland, the typical upland forest is mixed second growth with varying amounts of red spruce, balsam fir, white pine, hemlock, red maple, sugar maple, red oak, birches, beech, and bigtooth aspen. The climate of the northern portion of the watershed is more continental.