Boston Harbor Lighthouse
November 02, 2007
About nine miles (14 km) from downtown Boston, America's first lighthouse, Boston Light, marks the junction between Boston's outer harbor and the Atlantic Ocean. Rebuilt after being destroyed by the British as they were forced to evacuate Boston in 1776, it has guided mariners for almost 250 years.
Boston Harbor is characterized by a confusion of islands that are remnants of much earlier calamities, the rifting of the Avalonian microcontinent and subsequent waves of glaciation. Approximately 500 million years ago, Avalonia formed as a volcanic arch microcontinent on the fringe of Gondwana (then located well down in the southern hemisphere), marking a subduction zone. As the Iapetus Sea closed, Avalonia rifted and then broke free of Gondwana, migrating to eventually impact Laurentia and fuse with what would in time become New England.
The 34 Boston Harbor islands consist of a foundation of intrusive plutonic and sedimentary rock and a cap of glacial tills. The inner harbor group, more protected from Atlantic storms, retains their drumlin caps. However, the outer harbor islands, exposed to Atlantic storms for thousands of years, have largely lost their caps of glacial till, exposing their underlying basalt and sedimentary foundations. Little Brewster Island, the platform upon which Boston Light rests, is the outermost of the harbor islands, and it's been scoured clean of glacial materials by countless Atlantic storms. It consists largely of a basalt dike and some Cambridge Slate, and provides a stable platform for America's first lighthouse. Photo taken on November 18, 2006.