Radio echo sounding data reveal intensely deformed structures deep within the northern Greenland Ice Sheet. The geometry of these structures has been well studied, but their physical properties remain poorly understood. Here we investigate their scattering characteristics using radar swath imaging. Strong, diffuse backscattering implies that these features are not simply deformed meteoric layers, but instead contain distinct horizons of subglacially sourced debris. In many places, this debris is transported more than 1,000 m above the bed, altering ice strength and concentrating deformation in ways not captured by current ice-sheet models. These structures are widespread across northern Greenland, despite being absent in comparable glaciological settings elsewhere in Greenland and Antarctica. Based on their geometry, distribution and composition, we argue they formed as a result of transient basal thermal …