[8] The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level.
Most scientists believe that this climatic oscillation caused the major extinction event that took place during this time.
These formations were located at the very top of Ordovician deposits, and were dominated by a fauna which included brachiopods, trilobites, and other "shelly" or hard-coated animals.
In 1966, D. A. Bassett, Harry Blackmore Whittington, and A. Williams, writing in the Journal of the Geological Society of London, proposed a refinement to the Hirnantian Stage.
After considerable research, however, it was determined that no single faunal zone existed that could accommodate the upper stage of this division.
When the Hirnantian Stage began, the Earth's climate was hot and sea-levels were substantially higher than today.
As the climate cooled and glaciers formed during the early part of the Hirnantian, sea level dropped.
This drop dried up and exposed the extensive shallow-water continental shelves that existed throughout the world at that time, causing the extinction of large numbers of species who depended on this shallow water environment.
Zircon found in an ash deposit at the site was dated to 445.7 mya, with a margin of error of plus or minus 2.4 million years.
Radiometric dating placed zircon samples found in another ash deposit at 438.7 mya, with a margin of error of plus or minus 2.1 million years.
With these dates as a base, scientists were able to use biostratigraphic correlation techniques to determine close approximations for the timing of various events during the Hirnantian.
During the Hirnantian, much of the world's land mass was gathered into a supercontinent called Gondwana, which occupied extreme southern latitudes and covered the south pole.
Composed of modern Norway, Sweden, Finland, Denmark, northeastern Germany and Russia west of the Ural Mountains, this area ranged from the equator in the north to more than 30 degrees south latitude.
This consisted of what is now the southern parts of Britain and Ireland, and eastern coastal regions of Newfoundland, Nova Scotia, New Brunswick, and New England.