An expression for force balance is derived for the general case of gradients in longitudinal and lateral normal stresses and lateral shear stress. In order to consider horizontal glacial mechanics in Newton’s style of actions and reactions, the full stresses are partitioned into lithostatic and resistive, Rij, components. The lithostatic stress is the weight of ice above, and the horizontal gradient in lithostatic force on a vertical column is the familiar driving stress, which accounts for the horizontal effect of body or action forces. The horizontal resistive-stress components describe the reactions to this horizontal action of gravity. Force balance is

with horizontal coordinates x1, x2 and vertical z. The upper and bottom elevations are h and b, and τdi and τbi are driving stress and basal drag respectively. This describes net reaction due to normal resistance, lateral shear resistance, and basal drag resistance, and finally the action or driving stress. This equation is exact. Resistive stresses are simply linked to deviatoric stresses, and hence to strain-rates, through the flow law.