Rockfall trajectories are primarily influenced by ground contacts, causing changes in acceleration and rock rotation. The duration of contacts and its influence on the rock kinematics are highly variable and generally unknown. The lack of knowledge hinders the development and calibration of physics-based rockfall trajectory models needed for hazard mitigation. To address this problem we placed three-axis gyroscopes and accelerometers in rocks of various sizes and shapes with the goal of quantifying rock deceleration in natural terrain. Short ground contacts range between 8 and 15 ms, longer contacts between 50 and 70 ms, totalling to only 6% of the runtime. Our results underscore the highly nonlinear character of rock-ground interactions.