A succession of basaltic lavas, volcaniclastic sediments, and lignite beds in the Holmatindur region of eastern Iceland provides the means for tying magnetic reversal stratigraphy and a record of major North Atlantic cooling to a precise radiometric timescale. A prominent part of the section is the Holmatindur clastic bed, which is up to 66 m thick and can be traced 80 km along strike. The geological significance of this unit lies in its composition and associated thin lignite seams. The volcaniclastic composition is basaltic (in contrast to the common rhyolitic/dacitic composition of tuff units in the area), and hyaloclastic material is dominant. Hydroexplosive volcanism most likely produced the hyaloclastite, through subglacial eruptions within the paleovolcanic zone. Pollen assemblages from the lignite beds indicate a dramatic climatic deterioration, from subtropical to cool temperate conditions, through these sediments. Feldspar crystals separated from the Holmatindur clastic bed were determined to be from 10.72 (¿0.16) Ma, from 40Ar-39Ar incremental heating experiments. We correlate the Holmatindur cooling event with a relative maximum in benthic foraminiferal Δ18O (Zone Mi6 of Miller et al. <1991>), which has been interpreted as an episode of ice accumulation within the mid-Miocene cooling period, as well as with pulses in ice-rafted debris supplied to deep water sites in the North Atlantic Ocean, a global sea level drop of 70 m, and submarine canyon cutting, all of which occurred within the early part of magnetic anomaly 5 normal (C5n). While strong evidence exists for mid-Miocene ice sheets in Antarctica, northern hemisphere glaciation is thought to have started much later. The Iceland sites studied here demonstrate that significant ice accumulation occurred, albeit intermittently, as early as late middle Miocene time in the North Atlantic Ocean. From radiometric dating of Tertiary lavas that lie below and above the well-established lower boundary C5n, we conclude that the onset of this long normal polarity period occurred at 10.94 (¿0.16) Ma. The new data are compatible with the Cande and Kent <1995> interpolated estimate for this boundary (10.95 Ma) but are significantly older, or more precise, than previous direct dating by conventional K-Ar methods. ¿ 1998 American Geophysical Union