A study is presented of the general response of an ice age ice sheet model recently developed by Weertman (1976). The model is subjected to fluctuating insolation on time scales of the order of the variations in the earth's orbital parameters, which happen also to be the characteristic internal time scales of the ice sheets themselves. Because of this equivalence in time scales the model displays a wide range of behavior for reasonable parameter values, ranging from periodic growth and decay of ice sheets alternating with interglacial intervals to growth of large permanent ice sheets of periodically fluctuating size. The model displays nonlinear behavior analogous to a rectifier: as a consequence, a possible explanation of the 100,000-year cycle observed in geological climate time series is suggested.