Spectral roughness offers a significant potential for understanding the evolution of glaciated landscapes. Here, we present the first roughness study combining a high-resolution (250 to 500m) DEM of a large land-terminating section (12,000km2) of the Greenland Ice Sheet with the topography of the proglacial area. Subglacial roughness shows a directional dependence with consistently lower values in the ice flow direction compared to the acrossâflow direction. We find a correlation between low basal roughness, fast ice flow, and subglacial troughs. The northern part of the subglacial study area has an undulating topography with variable roughness, resembling the landscape in the proglacial area. In this area, there is a glacially eroded, overdeepened trough with bed elevations 510m below sea level, consistent with warm ice and a well-lubricated bed. The southern part of the subglacial study area has higher bed elevations and higher roughness than the northern part, possibly because the bedrock consists of hard granitic gneiss as in the adjacent proglacial area. The subglacial troughs, which have been eroded to various extents, are aligned with geological weakness zones suggesting a preglacial origin. In general, there is a major geological control on the distribution of bed variability.