Geology is destiny |
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By twenty thousand years ago, the latest continental glacier had pushed south from Labrador and across eastern North America (including Boston) to create lower Cape Cod and Long Island as its terminal moraines. The Cape and scattered islands also created by the ice protected the western end of Massachusetts Bay to provide the many excellent harbors that attracted the first European settlers.
Boston bedrock is mechanically strong metamorphic rock (quartzite, "puddingstone," and phyllite), able to support tall buildings without the need for special foundations. Atop the bedrock is gravel, sand, and the Boston Blue clay (powdered rock made by the grinding ice) left when the ice melted back about 15-12,000 years ago. Minor re-advances of the ice pushed the gravel up into hills of which Beacon with the other two peaks of the "Tri-mount" were the highest and most significant. Even after the ice had melted away from Boston, a lot of water still remained in glaciers worldwide so that sea level was more than 10 m (35 ft) lower. The blue clay, left exposed to the weather, oxidized into a hard-pan surface that now supports, through wooden pilings, much of old Boston. Gradually, the sea rose to fill the shallow basins and leave the oddly shaped Shawmut peninsula surrounded by tidal flats. Sand and gravel have porosity (empty space between the grains)that holds water from rain falling onto the gravel hills (like Beacon, for example). This water flows slowly down to emerge as swamps or springs on the lower slopes. The Great Spring is one reason Boston was established here. Earthquakes Boston is in a stable continental plate interior, far from any active boundary where stress accumulates because of plate movements (consider LA, for example). Nonetheless, stress does collect in plate interiors and is released by earthquakes felt in Boston. The good news is that intra-plate earthquakes are unlikely to generate destructive sea waves (tsunami) but the bad news is that unconsolidated materials tend to become unstable when shaken and much of Boston is built on land made of gravel dumped atop marine mud. In 1755 (18 days after the great Lisbon earthquake), the largest recorded local seismic event toppled many chimneys and broke off the weather vane atop Faneuil Hall. According to a contemporary account, "In Boston, twelve to fifteen hundred chimneys toppled, gable ends of brick buildings broke off and fallen bricks blocked the streets. According to John Hyde, a Boston writer, damage was particularly heavy "on the low, loose ground made by encroachments on the harbor …"(website, no citation) but there are no specific reports of damage to Faneuil Hall or to the other brick buildings in the Blackstone Block similarly erected on unconsolidated glacial till and outwash. Engineers are concerned that most Boston buildings were constructed before earthquake mitigation was required (or even considered). Seismic energy travels fastest in rock and slowest in mud so seismic energy will accumulate in the made land around Boston focusing it on the older buildings and shaking them down. Another rarely considered danger is the "slate storm" likely to occur when the old roofing slates now held in place mainly by friction are subject to sideward accelerations. |