Chalk exhibits mechanical properties ranging from those akin to a stiff soil to those akin to a strong rock, depending on its diagenetic and deformation history. Two processes are identified which lead to this hardening of chalks: consolidation, which leads to a reduction in porosity to about 40%; and pressure solution/cementation which will occur only after consolidation has increased the effective stress. Both of these processes are restricted by low permeabilities so that during hardening it is necessary for the deformation to maintain permeable pathways to dissipate any excess pore fluid pressure. This is facilitated by fracturing or by the development of open stylolites which permit fluid flow. In the Isle of Purbeck (Dorset) a lateral gradient in chalk hardness across the Purbeck Axis reflects the increasing deformation of the chalk. Both consolidation and pressure solution/cementation have occurred, with complete cementation being restricted to the most highly strained chalk which is also the most fractured. The Purbeck Chalk provides a model for both diagenetic and localized deformation hardening in chalks generally.