It has always seemed bizarre that there has been so little evidence for glaciation in Europe before the late Middle Pleistocene, whereas there were clearly extensive earlier glaciations in North America, even though their precise dating is unclear.
It is coincident with the bottom of a marly layer resting atop a sapropel called MPRS 250 on the southern slopes of Monte San Nicola in formed repeatedly on the landmasses and has been informally referred to as the “Great Ice Age.” The timing of the onset of this cold interval, and thus the formal beginning of the Pleistocene Epoch, was a matter of substantial debate among geologists during the late 20th and early 21st centuries.
The current ice age, called the Quaternary glaciation, has seen more or less extensive glaciation on 40,000 and later, 100,000 year cycles.
Originally, the glacial and interglacial periods of the Quaternary Ice Age were named after characteristic geological features, and these names varied from region to region.
In fact, the planet seems to have three main settings: “greenhouse”, when tropical temperatures extend to the poles and there are no ice sheets at all; “icehouse”, when there is some permanent ice, although its extent varies greatly; and “snowball”, in which the planet’s entire surface is frozen over.
Why the ice periodically advances – and why it retreats again – is a mystery that glaciologists have only just started to unravel.