Data obtained during the NASA Stratosphere Troposphere Exchange Project (STEP) Mid-Latitude Field Experiment displayed laminae of ozone, water, and condensation nuclei in the stratosphere in association with a mid-latitude stratosphere-troposphere folding event. Danielsen et al. (1991) constructed cross sections of these observations, and interpreted these quasi-horizontal laminae as evidence of ultra-low frequency gravity waves. We use a new technique to show that these laminae could have resulted from differential advection, rather than transport by ultra-low frequency gravity waves. This new technique uses reverse domain filling back trajectories on multiple isentropic surfaces in conjunction with modified potential vorticity to reveal the qualitative details of the constituent laminae.