  We present evidence for the existence of substructure in the "relaxed
appearing" cold front clusters Abell 2142 and RXJ1720.1+2638. The detection of
these substructures was made possible by comprehensive multi-object optical
spectroscopy obtained with the Hectospec and DEep Imaging Multi-Object
Spectrograph instruments on the 6.5m MMT and 10m Keck II telescope,
respectively. These observations produced 956 and 400 spectroscopically
confirmed cluster members within a projected radius of 3Mpc from the centers of
Abell 2142 and RXJ1720.1+2638, respectively. The substructure manifests itself
as local peaks in the spatial distribution of member galaxies and also as
regions of localized velocity substructure. For both Abell 2142 and
RXJ1720.1+2638, we identify group-scale substructures which, when considering
the morphology of the cold fronts and the time since pericentric passage of a
perturber estimated from the cold front radii, could plausibly have perturbed
the cluster cores and generated the cold fronts observed in Chandra images. The
results presented here are consistent with cold fronts being the result of
merger activity and with cold fronts in relaxed appearing clusters being due to
minor merger activity.
