Spatially limited, strong acoustic reflectors, interbedded with turbidite sediments, appear in abundance in the region flanking the Dellwood Knolls, northeast Pacific, where turbidite sediments nearly cover a newly formed spreading center near the continental margin off central British Columbia. In the median valley, similar reflective layers are seen within the sediments as well as on the sediment surface, and the conclusion has been made that all of the strong reflections are from extensive basalt flows which have had their source at the Dellwood Knolls and have been subsequently buried by rapid sedimentation. The smooth, gently sloping turbidite sediment surface appears to provide an ideal base over which flows can travel easily; some of the flows are seen as far as 60 km from their source. Evidence for the existence of extensive flows is also found on the Juan de Fuca Ridge, where, at one location, the regional topographic relief of the axial valley floor along a 30-km section of the ridge axis is only 30 m. Such extreme low relief must be due in part to extensive basalt flows. A simple model for the cooling of an underwater flow yields the general dependence of the flow extent on such parameters as the flow thickness and initial temperature. As an example, a flow 1 m thick can probably travel several kilometres if erupted at 1,250 °C; even the most extensive flows observed need be only about 3 m thick if erupted at this temperature.