The term ‘sikussak’ has a complex and disordered history of use in glaciological literature, and no dedicated studies of sikussak characteristics have yet been carried out. Using ATM elevation data from the NASA Operation IceBridge campaign, profiles of pro-glacial sikussak and ice melange are derived. Sikussak structures are found to be approximately 15 – 150 m thick and 1 – 35 km long, with surface slopes in the range ⁻0.1 – ⁻0.7°. These structures are therefore larger than is generally perceived, and may have a greater impact on the sensitive terminal zone of tidewater glaciers than previously considered. This study aims to characterise pro-glacial sikussak and melange structures and their importance at the calving front of tidewater terminating glaciers using a dataset of 13 glaciers around the Greenland Ice Sheet. Based on observations from these data, a classification system is proposed for the different pro-glacial structures found in Greenlandic fjords, and the geographical location of these various structures is delimited using a fast-ice index based on the ratio between positive and freezing degree days. The primary controls affecting sikussak morphology are investigated, and a hierarchy of glacio-dynamic, environmental and topographic factors are identified. Finally, it is proposed that there exists a stabilising feedback between rapid glacier flow and sikussak formation, and that this may be threatened under future climate change.