Flutes are widespread across the forefield of a polythermal valley glacier Midre Lovenbreen, Svalbard, where they consistently form on the downglacier face of cobble to boulder-sized subglacially-transported clasts ploughed into proglacial till. The width of flutes at Midre Lovenbreen is constant along their long-axes and proportional to the size of their associated subglacially-transported boulders. Frozen flutes are observed subglacially where they protrude into the debris-rich basal ice layer of the glacier. Particle-size analysis shows that flutes are depleted in gravel-sized particles relative to adjacent subglacial till. Ice sampled from flutes is enriched in delta 18O relative to overlying glacier ice and debris-rich basal ice. These properties are interpreted to reflect the formation of flutes by the squeezing of till into basal cavities associated with boulders beneath warm-based ice in the interior of the glacier. It is hypothesized that localized freeze-on of flute sediments, within a broader zone of warm-based ice, occurs within cavities due to either (i) a 'heat-pump' effect operating across these boulders leading to the development of a cold patch within the cavity, and/or (ii) fluctuations in cavity length caused by slip-stick glacial motion. Once incorporated into the basal ice layer, the frozen flute sediment would then be transported away from the boulder by ice flow, allowing the cavity to remain continuously open and the flute to propagate downflow.