A simple, untuned constant sedimentation rate timescale developed using three radiometric age constraints and eleven Δ18O records longer than 0.8 Myr provides strong support for the validity of the SPECMAP timescale of the late Quaternary <Imbrie et al., 1984>. In particular, the present study independently confirms the link between major deglaciations (terminations) and increases in northern hemisphere summer radiation at high latitudes and shows that this correlation is not an artifact of orbital tuning. In addition, the excess ice characteristic of late Quaternary 100-kyr climate cycles typically accumulates when July insolation at 65 ¿N has been unusually low for more than a full precessional cycle, or >21 kyr, and once established does not last beyond the next increase in summer insolation. Thus, the timing of the growth and decay of large 100-kyr ice sheets, as depicted in the deep sea Δ18O record, is strongly (and semipredictably) influenced by eccentricity through its modulation of the orbital precession component of northern hemisphere summer insolation.¿ 1997 American Geophysical Union