Current understanding of ice dynamics predicts that increasing availability and variability of melt water will have an impact on basal motion, and therefore on the evolution and future behavior of the Greenland Ice Sheet. We present measurements of ice deformation, subglacial water pressure and surface velocity that show periodic and episodic variations on several time scales (seasonal, multi-day and diurnal). These variations, observed with GPS and sensors at different depths throughout the ice column, are not synchronous but show delayed responses of ice deformation with increasing depth, and basal water pressure in anti-phase with surface velocity. With the help of a Full-Stokes ice flow model these observations are explained as ice motion in a caterpillar-like fashion. Caused by patches of different basal slipperiness, horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns. Variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior. Ice flow in the ablation zone of the Greenland Ice Sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season, leading to caterpillar-like ice motion superposed on the classical shear deformation.