A Chronological Survey of Sedimentary Landforms in the Continental United States/Ordovician
488.3± 1.7 Ma to 443.7±1.5 Ma
|Mean atmospheric O2 content over period duration||ca. 13.5 Vol %
(68 % of modern level)
|Mean atmospheric CO2 content over period duration||ca. 4200 ppm
(15 times pre-industrial level)
|Mean surface temperature over period duration||ca. 16 °C 
(2 °C above modern level)
|Sea level (above present day)||180m; rising to 220m in Caradoc and falling sharply to 140m in end-Ordovician glaciations|
The Ordovician period opened with no sedimentary break from the Cambrian, but is nevertheless distinct because the fossil assemblage changes drastically. Many Cambrian marine species disappeared or were greatly reduced in numbers in what has been called the Cambrian-Ordovician Extinction Event (Wikipedia article). The most useful group for dating the Cambrian-Ordovician boundary is the Brachiopods. This group was dominantly composed of inarticulated forms (lacking a specific hinge structure between the shell halves) during the Cambrian, but the relative abundance is reversed at the beginning of the Ordovician. The hinge may have provided some added protection against predators, which were now beginning to appear in the fossil record.
The Ordovician period opened with the Cambrian Sauk Seas still dominating the landscape. The Sauk transgression reached its maximum extent during Early Ordovician time, then began a slow regression that culminated in the complete removal of the epeiric sea at the end of the Early Ordovician. It is thought that this regression represents a period of cratonic uplift due to tectonic forces. The regression of the Sauk Seas left behind a blanket deposit of shoreline sandstones that represent erosion of the newly emerged craton.
This exposure of the craton to subaerial erosion appears to have lasted for a very long time. No depositional sequence exists by which we may determine exactly how much time is missing, but the fossil record suggests it was many millions of years. During this time, what little geographic relief the craton possessed was eroded down, and the clastic remnants of that erosion are present in the sand layer. The flat, sand covered surface was extensively reworked by erosional forces, and eventually became quite uniform in composition across wide areas of the craton.