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New paired carbonate and organic-carbon isotope analyses from Nevada, USA, together with a consideration of the effects of mountain-building and ice-sheet coverage of the continents on atmospheric pCO₂, lead to a new hypothesis for the cause of the Late Ordovician glaciation. We suggest that the Taconic orogeny, which commenced in the late-middle Ordovician, caused a long-term decline in atmospheric pCO₂ through increased weatherability of silicate rocks. Ice-sheet growth was triggered when pCO₂ decreased to a threshold of ∼10× present atmospheric level and proceeded by positive ice-albedo feedback. In the midst of glaciation, atmospheric pCO₂ began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets. At first, this enhanced greenhouse effect was overcompensated for by ice-albedo effects. Ultimately, however, atmospheric pCO₂ …