Crowleys Ridge, is an erosional remnant of 40-to 50-million-year-old sedimentary rock in the upper Mississippi River Embayment. The ridge formed late in the Pleistocene as the river, gorged with glacial meltwater, continually shifted course and broadened its floodplain at the expense of Tertiary rock forming its banks. Crowleys Ridge is located near the site of the great 8+ magnitude New Madrid earthquakes of 1811-1812. - USGS