A narrow strip of land connecting the [Shawmut Peninsula](https://en.wikipedia.org/wiki/Shawmut_Peninsula) to the mainland. The neck was originally only 120 feet wide at high tide. The first settlers built an earthen wall and a town gate in 1631, with a gallows just outside. The Charles River tidal flats were filled beginning in the 1820s, creating the South End, Back Bay, and Fenway.