We study the formation and evolution of localized geometrical defects in an indented cylindrical elastic
shell using a combination of experiment and numerical simulation. We find that as a symmetric localized
indentation on a semicylindrical shell increases, there is a transition from a global mode of deformation to
a localized one which leads to the condensation of curvature along a symmetric parabolic defect. This
process introduces a soft mode in the system, converting a load-bearing structure into a hinged, kinematic
mechanism. Further indentation leads to twinning wherein the parabolic defect bifurcates into two defects
that move apart on either side of the line of symmetry. A qualitative theory captures the main features of
the phenomena but leads to further questions about the mechanism of defect nucleation