jats:titleAbstract</jats:title> jats:pThe Skaergaard Intrusion of East Greenland solidified as a closed system, with the development of progressively more fractionated material at the roof, floor and vertical walls of the magma chamber. We argue, using field observations of the exposed western margin together with detailed microstructural and geochemical analysis, that the mush on the vertical walls of the Skaergaard Intrusion reached a maximum thickness of ~180 m, and was highly unstable. Material was lost both continuously, due to non-retention of poorly consolidated material, and during episodic collapse events. The almost complete absence in the wall rocks of material formed in LZc times (following the saturation of the bulk magma in Fe-Ti oxides) was likely to have been caused by a collapse event, perhaps related to faulting of the actively extending Greenlandic margin. A second major collapse of the MBS occurred during the later stages of solidification, with the greatest loss of material occurring in a region of the wall with an unusually thick mush, creating a transient and localised increase in the thickness of the floor mush. This work demonstrates the importance of vertical walls in supplying loose, disaggregated material to form mobile magmatic slurries that may contribute to floor cumulates or be entrained and erupted in long-lived systems.</jats:p>