Direct evidence of the late Paleozoic glaciation of Gondwana from glacial deposits suggests that geographically extensive continental glaciation began some time in the Namurian (Late Mississippian). However, the timing and characteristics of the onset of glaciation are poorly understood because of a lack of reliable paleontological control and reworking of initial glacial deposits by subsequent glacial advances. Indirect evidence of glaciation preserved in unconformity-bounded, low-latitude ramp sequences in the Illinois basin, United States, suggests that geographically extensive continental glaciation of Gondwana actually began in the late Visean. An abrupt change from carbonate-dominated sequences bounded by disconformities with little evidence of erosion to mixed carbonate-siliciclastic sequences bounded by unconformities with deep incised valleys was likely produced by a three-fold increase in the magnitude of eustatic sea-level fluctuations. The increase in the magnitude of sea-level fluctuations was likely driven by an equally abrupt increase in ice volume and marks the onset of the geographically extensive late Paleozoic glaciation of Gondwana. A possible explanation for the rapid onset of glaciation is the closing of the equatorial seaway between Laurussia and Gondwana. Closing of this seaway would have led to an abrupt change in oceanic and atmospheric circulation patterns that could have initiated major continental glaciation in the Southern Hemisphere.