Chalk rock masses contain natural discontinuities that are largely parallel and perpendicular to the ground surface. In terms of foundation loading, it is the normal deformation of the horizontal discontinuities, which are of particular interest. This paper considers the influence of discontinuity contact area on the deformation behaviour of chalk. A suite of unconfined compressive strength tests is described, which comprises an intact specimen and five discontinuous specimens. The contact area at the discontinuity ranges from 57% of the initial cross-sectional area of the specimen to 17% contact area. The deformation processes are modelled analytically and the experimental results are compared with the predicted values. The experimental results show excellent agreement with the proposed model and identify a critical contact area, between 44% and 51%, which determines the mode of deformation behaviour.