The Early Palaeozoic Icehouse was a cool period that interrupted the greenhouse temperatures of the Ordovician and Silurian periods, culminating in the Hirnantian glaciation and the Ordovician extinction event. The icehouse was formerly thought only to consist of the Hirnantian glaciation itself, but has now been recognized as a longer, more gradual event.
References
Early Palaeozoic Icehouse Wikipedia(Text) CC BY-SA