2D shell crossing in Eulerian x − y space (top row) along with a q_x axis slice through the shell-crossing point q_c in q_x − x space (bottom row) to help visualize the folding of the CDM sheet. Left column: Snapshot during the single-stream flow prior to shell crossing (t < t_c). Middle column: Moment of shell crossing (t = t_c). A catastrophe event marked by the appearance of an A₃⁺ singularity (red dot) seeds the 2D pancake-like structure, shown in the right column (t > t_c). The boundary of the pancake encompassing the multi-streaming region is defined by A₂ singularities (blue), which grows along the A₃⁺ spine (green).