Understanding the material properties and physical conditions of basal ice is crucial for a comprehensive understanding of Antarctic ice-sheet dynamics. Yet, direct data are sparse and difficult to acquire, necessitating geophysical data for analysis. Here, we employed high-resolution ultra-wideband radar to map high-backscatter zones near the glacier bed within East Antarctica's Jutulstraumen drainage basin. Our results revealed that the basal ice in an area of ~ 10,000 square kilometers is composed of along-flow oriented sediment-laden basal ice units connected to the basal substrate, extending up to several hundred meters of thickness. Three-dimensional thermomechanical modeling suggests these units formed via basal freeze-on of subglacial water originating further upstream. Our findings suggest that basal freeze-on, and the entrainment and transport of subglacial material play a significant role for an accurate representation of material, physical, and rheological properties of the Antarctic ice sheet's basal ice, ultimately enhancing the accuracy and reliability of ice-sheet modeling.Here, we publish locations and thickness of high-backscatter zones at the onset of the Jutulstraumen glacier in East Antarctica.