Iceberg calving is a major contributor to Greenland’s ice mass loss and to sea level rise. Pro-glacial mélange (compressed ice fragments) may be tightly packed in the long, narrow fjords that front many marine-terminating glaciers and can reduce calving by buttressing. The effect is important for modeling glacier dynamics in Greenland. However, data limitations have hampered a quantitative understanding. We describe a new radar-based approach to map time-varying elevations near the mélange-glacier interface that provides a factor of three or more improvement in precision, and apply the technique to the terminus of Jakobshavn Isbræ, a major Greenland outlet glacier. The new data allow mélange changes to be quantified with high spatial and temporal resolution. Over a one-month period from late May to late June 2016, the glacier experienced essentially no major calving, and was buttressed by a thick mélange wedge. Large-scale calving began once mélange mass within 7 km of the glacier front decreased by >40%. And the total buttressing force decreased by ~1 × 107 N m-1 (the unit is force per meter lateral width) during a 13 day period with continuous observations by a terrestrial radar interferometer. To our knowledge, these are the first observations of a near-field, time-varying pro-glacial mélange wedge. The observations provide direct evidence that tightly-packed mélange reduces calving, and place quantitative constraints on the process.