Geology of Boston, Massachusetts, United States of America

Abstract

The geology of Boston directly influenced the founding of Boston. It provided the early settlers with a safe harbor, protection from their enemies, and life-giving water. Initial building was confined to the high ground associated with the Beacon Hill Moraine on the 319-hectares (789-acre) Shawmut Peninsula, as Boston was then called. The need for more space forced the colonists to look upon the surrounding tidal marshes as areas to fill and build upon. The lowering and removal of the hills of Boston provided suitable material for the filling of the downtown area. As this source was depleted, additional material was brought in from the surrounding towns. The regional geologic setting of Boston appears to be the most varied and complex in the eastern United States. Almost all periods from Precambrian to Holocene are represented in the vicinity of the city. Boston lies in a fault-bounded basin dropped into a late Precambrian batholithic complex just southwest of a zone of very large faults. The basin contains a wide variety of slightly metamorphosed rock that, in many places, has highly irregular stratigraphic and structural relations. Bedrock consists of mostly argillite and conglomerate with a large variety of fine-grained intrusive and extrusive igneous rock, occurring in and around the basin. Overlying the rock are glacial deposits of such complexity that their study ultimately led to the development of soil mechanics as a science in this country. Initial urban development caused the first confrontation between the engineer of early colonial times and the complex geology underlying Boston. The need for deep foundations became evident, and the large buildings of historical time were founded on wood piles. This early foundation engineering culminated in today9s modern skyscrapers supported on a variety of foundation systems including piles, caissons, and mats. Geologic constraints are essentially limited to certain geotechnical properties of the overburden, ground-water conditions, and the sporadic occurrence of altered bedrock. Boston lies in an area of moderate earthquake hazard and has experienced damaging earthquakes in 1638, 1663, 1727, and 1755. The 1755 earthquake was the largest one from a nearby source off Cape Ann. It produced an intensity of VII in the city and possibly greater near the harbor. An estimated maximum credible earthquake may produce one intensity greater. The very extensive areas filled and built upon since 1755 have increased the potential for damage from earthquakes. The development of the City of Boston into a large metropolitan area in the 19th Century made Boston a leader in the development of a metropolitan water and sewerage system. Boston also led the nation in developing a structured solid waste management program. Beginning in 1895, Boston now has over 125 km (77.4 mi) of subway tunnels which include segments of historical significance such as the first subaqueous transit tunnel constructed under air pressure, and the nation9s oldest subway tunnel.