Mechanical collapse of recessed features of relief on elastomeric elements for soft lithography represents an important phenomena for this lithographic technology. By comparing computed and measured shapes of partially collapsed structures, we show that the dominant mechanism for collapse is surface adhesion between the elastomer and substrate, for typical materials and processing conditions. In particular, the shapes obtained using models that account for surface adhesion agree well with the experimentally measured shapes. Electrostatic forces may contribute to this process, but they do not dominate. The weight of the elastomer has essentially no effect.