The Mississippian is a period in the geologic timescale or a series of the geologic record. It is the earliest/lowermost of two divisions of the Carboniferous period lasting from roughly 359 to 318 Ma (million years ago). As with most other geochronologic units, the rock beds that define the Mississippian are well identified, but the exact start and end dates are uncertain by a few million years. The Mississippian is so named because rocks from this age are exposed in the Mississippi River valley.