Feni and Gardar ridges are two major North Atlantic sediment drifts-positive sedimentary accumulations over 1600 m thick presumed to result from sediment distribution by bottom water circulation since Eocene-Oligocene time. At Deep Sea Drilling Project Sites 610 and 611 we penetrated the Miocene to Holocene sequences of these drifts, and we looked for depositional changes across sediment wave fields on their surfaces with a series of offset holes. The sediment waves, over 20 m in height and over 2 km in wavelength, are morphologically more complex than hitherto recognized. Around the drill sites, waves characterize the upper 100 to 200 m of seismic sections, but we found no evidence of wave migration in these records. Lithologies in the sediment waves are fundamentally pelagic. Few current-produced sedimentary structures or hiatuses were found. Glacial cycles allow detailed wave-to-trough correlations, which show that since 2.4 Ma, little or no wave migration could have occurred. Sedimentation rate differences between wave and trough locations suggest that wave migration did occur in the Pliocene at the Gardar Ridge site. A review of possible sedimentary processes and parameters indicates that internal waves in a stratified water column might be important for the initiation and migration of sediment waves. Changes in sedimentary regime on the drifts seem to be linked to interaction between bottom water masses from northern (Norwegian Sea Overflow Water-NSOW) and southern (Antarctic Bottom Water-AABW; and North Atlantic Deep Water-NADW) sources and not to changes in intensity of NSOW alone. Regional seismic reflectors drilled at the Feni Ridge site are linked to oceanographic changes: "R2" of early Miocene age appears to signify a silica productivity event; "Rl" of late Miocene age could be linked to the isolation of the Mediterranean, which may have affected the NADW slope current. Our interpretation of the sedimentary history of the drifts is that during the preglacial period NSOW was probably the dominant bottom water mass over the drifts and the sediment waves were actively migrating. We suggest that the influence of NSOW lessened with the onset of the glacial-interglacial periods. Since that time, bottom currents have simply "maintained" the morphology of the now largely relict wave fields.