The Ordovician Period is stratigraphically younger than the Cambrian Period and stratigraphically older than the Silurian, which overlies it.
The Ordovician is now dated between 488.3 and 443.7 Ma (million years ago), which makes its duration 44.6 million years (Webby, 2003).
Paleontologists have developed an equally well-defined chronostratigraphic framework for use in the cratonic rocks of North America.
The Ordovician, like most periods, has traditionally been sub-divided into Early, Middle and Late epochs, and Lower, Middle and Upper Series, but in practice, this classification is of little use except for the application of broad general patterns.
In order to make a more useful and practical time-scale the Ordovician has been sub-divided into distinctive, well-defined intervals of time often referred to as series and stages.
The British classification system is more often used and represents the preferred chronostratigraphic system for the Ordovician.
The relationship between the British and North American stage nomenclature is provided in the figure above.
These include: a) the basal Trenton Unconformity which is accentuated (and of longer duration) in the central Mohawk Valley in the neighborhood of the Canajoharie Arch; b) the upper Trenton Lower Utica disconformity centered in the Western Mohawk Valley region in the vicinity of Middleville, New York; and c) the top Schenectady disconformity centered in the region to the east of Little Falls New York; 2).