The Seawall area, along Route 102A as it circles between Bass Harbor and Southwest Harbor, provides Acadia National Park visitors their access to the ocean on the western half of Mount Desert Island. One of the park’s two campgrounds, as well as a picnic area, is located just to the south of the natural seawall created here over time as storm surges have stacked up large rocks along the shore.
The exposed bedrock at the seawall is much different from the rugged granite cliffs on the eastern side of the island. It’s an extrusive igneous tuff that originated some 400 million years ago when volcanic eruptions produced clouds of light gray layers of fine ash that settled on the ocean floor, and then was transformed by pressure and heat. Geologists call these stratified rocks the Cranberry Island Series. (The bedrock of the Cranberry Islands just offshore from the seawall also consists of the Cranberry Island Series. Duh.)
The dominant tree along this shoreline is white spruce, and the interior of the Seawall area mostly consists of peatland created by maritime weather: fog, clouds, mist, and rain. Both spruce and peatland generally are found only farther to the north, evidence again of Mount Desert Island’s unique location between the land and the sea, and between the northern Arctic and southern temperate zones.
FUN FACTS: Geologists may refer to the Cranberry Island Series (also the Bar Harbor and Ellsworth formations) as ‘weak’ rock. By this they mean not that the rock really is weak, but that it’s weak compared to the granite more commonly found on the island. When the Cranberry Island Series was created way back in the Siluro-Devonian period, seaweed was the dominant form of plant life on Earth.