As mentioned above, there was a major climatic oscillation during the Hirnantian, which is believed to have caused a major mass extinction event.
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.
The younger date is from the early Silurian Birkhill shale formations.
As the climate cooled and glaciers formed during the early part of the Hirnantian, sea level dropped.
Estimates of the depth of this drop range from more than 50 meters (based on studies in Nevada and Utah) to more than 100 meters (from studies in Norway and the United Kingdom).
Both dates are from the Dob's Linn area in the United Kingdom.
The older one comes from the local Hartfell shale formations.
While it is not usual practice for scientists to formally divide geologic time below the level of a stage (though sub-stages are known in some instances), zones of biological distinction (called biozones) provide a generally recognized way of defining divisions at smaller levels.
Biozones represent a period of time during which a specified species or group of species existed and left a fossil record of their presence.
More recent research indicates a worldwide reduction in sea level of approximately 80 meters.