As ice retreated from southern coastal Maine from 15,000 to 13,000 years ago, the ocean inundated coastal areas where the earth’s crust had been temporarily depressed by the weight of glacial ice. A thick veneer of glacial-marine clay and silt – the Presumpscot Formation – was deposited in coastal Maine lowlands. Due to its fine-grained character, shear waves from seismic events travel slowly t...