Overlap sections of the Somerset and Weston Canal cores. The colors correspond to approximately the colors of the mudstones. The black bars represent normally magnetized rock; the white bars represent reversely magnetized rock (the polarity--normal or reversed--of the Earth's magnetic field periodically switches; the polarity at the time of rock formation is recorded by sedimentary or igneous rocks containing magnetic minerals). Note that stratigraphic units are thicker in Weston Canal compared to Somerset. Modified from Schlische (2003) based on data in Olsen et al. (1996a).