  Stellar shells are low surface brightness features, created during nearly
head-on galaxy mergers from the debris of the tidally disrupted satellite.
Here, we investigate the formation and evolution mechanism of shells in six
dimensions (3d positions and velocities). We propose a new description in
action-angle coordinates which condenses the seemingly complex behaviour of an
expanding shell system into a simple picture, and stresses the crucial role of
the existence of different stripping episodes in the properties of shells.
Based on our findings, we construct a method for constraining the potential of
the host galaxy and the average epoch of stripping. The method is applicable
even if the shells cannot be identified or isolated from the data, or if the
data are heavily contaminated with additional foreground stars. These results
open up a new possibility to study the ancient merger that built the Milky Way
Galaxy's stellar halo.
