The growth and subsequent melting of the latest Ordovician-earliest Silurian glaciers in north and south Africa were responsible for a global transgression and regression. The shallow-water platform carbonates of the Cordilleran and Arctic regions of North America encompass a stratigraphic break apparently equal in duration to the glaciation. The transgression due to melting of the ice sheets was probably rapid. The break in sedimentation is without discordance and is recognized by subtle but abrupt lithologic changes and particularly by rocks with middle or late Llandoverian fossils resting directly on those with Late Ordovician fossils. Deep-water graptolitic shales, however, show a complete or nearly complete sedimentary sequence across the boundary.

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