Ranked after the late Palaeozoic and the late Cenozoic, the Ordovician- Silurian is the most extensive and intensive Cool Mode of Phanerozoic time. Glacial effects were felt predominantly in Africa, and in displaced terranes subsequently derived from there, and were also felt in South America. A major ice sheet developed in North and central Africa. The age of glacial deposits covers probably 35 m.y., as opposed to a minimum of 65 m.y. for the late Palaeozoic. Despite this widespread evidence of glaciation, it appears that the Ordovician-Silurian glaciation was limited to high-latitude land masses in the southern hemisphere; cooling effects are discerned with difficulty elsewhere.

Distribution and age of the glacials

Ordovician–Silurian glacial deposits, unlike those of the late Palaeozoic, are found in Gondwana and in regions commonly considered as parts of the Laurentia and Baltica blocks. The latter areas probably were attached to Gondwana in the early Palaeozoic and have since been rifted away to new sites in North America and Europe. The evidence of glaciation in these displaced continental fragments provided a strong incentive for revising continental reconstructions for the early Palaeozoic.

Glaciation at this time was centred on North Africa, the best documented deposits being those of the central Sahara region (Beuf et al., 1971). Tillites and associated glacial features occur over a wide area from Algeria to Libya and Mali. A total of four separate tillites and striated pavements showing generally northward flow have been recognized. Deposits of about the same age occur to the west and south-west in Morocco, Mauretania and Sierra Leone; these include both terrestrial tillites and glacial-marine strata.