A sophisticated physical model of the dry snow/firn densification process in ice sheets is proposed. Macroscopically, snow and firn undergo vertical uniaxial compression with non-zero deviatoric stresses and strain rates. The present mathematical description of densificntion includes dilatancy and "force-chain" effects in snow and develops previous concepts of ice-particle rearrangement by grain-boundary sliding and sintering by power-law creep under overburden pressure. Both densification mechanisms work together during the first snow stage until the closest packing of ice grains is reached at critical densities of 0.7-0.76 and the firn stage controlled only by the dislocation creep sets on. In addition to the ice-grain coordination number and the slope of the radial distribution function, a new structural parameter is introduced to account for grain bonding (agglomeration) effects. The model is constrained and validated on direct stereological observations of ice core structures and a representative set of snow/firn density-depth profiles covering a wide range of present-day climatic conditions (-57.5 to -10℃ and ice accumulation at 2.15 to 330 cm yr^[-1] ). Simple equations are derived for predicting the depth of pore closure in firn and the ice age at close-off. The paleoclimatic evolution of quasi-stationary density-depth profiles and close-off characteristics at Vostok Station (East Antarctica) are simulated and discussed. III. Firn densification, close-off and chronology